Beta'd by the amazing InsaneScriptist
I am not JK Rowling and do not own the Harry Potter franchise, though my take on the story is all my own.
Where it started
James Potter had discovered he couldn't have children halfway through his last year of Hogwarts.
It had been an accidental discovery: Slughorn had been talking about how potions accidents could have long-term health effects if not properly screened and treated on time, a tactic no doubt designed to cover his back should those students attempting to impress him accidentally mess themselves up while attempting the more challenging brews in their free time and not tell Pomphrey, but James had found himself wondering about all those prank potions the Marauders had brewed over the years and how some of them had gone wrong. The explosions had been entertaining afterwards but getting to know Lily had educated him about how Muggles hospitalised people who had been near fires and explosions due to them breathing in smoke and volatiles which damaged the lungs.
James hadn't been able to stop worrying about possibly damaging his friends, so he, Sirius and Remus had got Lily to help them learn the Diagnosis Charm, using a quill as the medium so that injuries and so-on would be written down rather than visible to the caster. It took a bit of modifying but Remus wasn't top of Arithmancy for nothing. They'd cast it on Remus first, since his furry problem would counter most other medical issues and would be the test of whether they'd got the charm right.
It did work, right down to the inoculations Remus had been given as a child, so James cast the spell on Peter while Remus did it on Sirius. Then Sirius cast it on James. Those copies were private, since Remus' copy had proved that yes, he had taken potion damage but his furry problem had fixed it before it set in. The other three Marauders didn't have lycanthropy so they were all bound to have taken some damage, which made James angry that Slughorn hadn't said something to this effect in first year. Even third year would have been soon enough. There had been no few accidents in class after all, a few with potentially dangerous ingredients the Potions Master hadn't made them all see Pomfrey about.
Peter didn't share what was on his paper, going very red then burning it, and none of the others really wanted to ask as they had their own diagnoses to focus on. Sirius discovered that an accident in third year had messed up his inhibitions and made him reckless, which sounded cool but was really bad considering there was a war on and he might get people killed and himself with them. It also destroyed Sirius' dream of becoming an auror, which depressed him for several weeks. James learned that the funny fizzy potion he had been tweaking for a prolonged bubbling phase that had abruptly boiled dry on him in fifth year while Sirius and Peter were in detention had over the past two years made him completely infertile. He had researched the plant that had induced the fizzy effect afterwards and learned that it was a common ingredient in Fertility Potions, so he hadn't been too bothered about it then. Going back to the Restricted Section after his diagnosis however revealed that when said ingredient was brewed with Gurdyroots the resulting potion was commonly fed to Magical Creatures to prevent them from breeding. James had breathed in a lot of fumes, so its effect on him was not at all surprising. Why wasn't this kind of thing mentioned in class!
James confessed this to Sirius privately after Sirius told him about his inability to become an auror or even a hit wizard. Sirius had been shocked, instantly shoving his own depression aside and spending the next five hours persuading his best friend not to break off his fledgling relationship with Lily Evans. James resisted his friend's arguments until Sirius swore an oath as James' adopted brother to assist him in having children in any way possible that didn't involve breaking eventual marriage vows, at which point the Potter heir folded like a cheap table and spent a further half-hour sobbing in relief at being able to not feel guilty about continuing his relationship with his girlfriend.
They never told Remus or Peter about either problem –it didn't feel right– but by the time they graduated James had found a potion that in normal people instilled mild paranoia but just made Sirius a bit more careful. It wasn't even addictive or poisonous over long periods of time, but did make latent paranoia more likely in the long run. Sirius just pointed out that for him that might be a good thing and James sighed, resigning himself to monthly brewing sessions in the basement with his brother for the rest of their lives.
Sirius on the other hand managed to sneak back into his childhood home in Grimmauld Place and steal a book on bloodline theft from the library. Since getting Lily to conceive without Sirius actually touching her would technically be done by means similar to bloodline theft, James was okay with it. If uncomfortable, as bloodline theft was wrong and doing it to his brother was doubly wrong, no matter how willing said brother was and that it had been Sirius' idea in the first place. The book also had details on blood-adoption that would pass goblin and magical scrutiny because House Black had of course tested and documented such things extensively in their murky past, so James would be able to make Lily's children his true heirs rather than just adopting them into his family.
Sirius hadn't wanted to tell Lily about James' problem, especially after finding out that for the potion to work the man involved had to be capable of fathering children –he'd hoped that with a potion James would be able to sire the kid because while ideal that kind of magical solution didn't seem to exist yet and neithr man had the genius to invent it– but James had insisted. Had told her right after proposing, in fact, so she could back out if she wanted. Lily had been horrified, but then her love, compassion and thirst for knowledge had kicked in and she'd hugged her fiancé and demanded to be in on the research. It had taken them a year to get everything just right in between fighting with the Order of the Phoenix, what with sorting out wills, custody, inheritance and so on as well as making sure the potion would work, but on the first of November a year after they graduated Lily drank the potion and a week later informed James and Sirius that they were going to be fathers. James did the blood adoption immediately: the ritual was a simple one often used in the middle ages to 'legitimise' children conceived through rape or love potions. A proper paternity test would reveal Sirius Black to be the baby's father, but the child would look a bit like James and register to the goblins as a Potter as well as a Black. If the goblins got curious then James just had to point out that his mother was a Black. As James was the only Potter left, that would make the baby his heir.
Picking out a name took forever though, even after Lily announced that the baby was a girl. Sirius was determined that the baby would be a Potter first, so tried to get James to pick the name. James insisted that Sirius should choose as actual father and Lily eventually got sick and tired of the stupidity and declared that the baby would be Dorea Rose Black-Potter. Sirius then risked life and limb pointing out that 'Rose' was rather plain as a name and had a book thrown at him. James quickly stepped in by suggesting 'Rosamund', which he eventually admitted was what his mother would have called him if he'd been born a girl. Sirius howled with laughter, Lily calmed down enough to giggle and James pouted at his wife and brother but secretly felt it was worth it. Even though Sirius charmed his hair pink for a week afterwards and called him 'Rosebud'.
When the baby was born, a lovely little girl with a shock of curly black hair and clear blue eyes that soon turned brilliant green like her mother's, James put her full name on the birth certificate for the family Vault but only put 'Rose Potter' in the Daily Prophet announcement. His daughter might be named after his mother but she had been a Black and Sirius' great aunt so 'Dorea' would be his daughter's 'Black' name. They were at war after all and keeping his precious baby girl safe was what mattered. If she would be safer as a Black –which she probably would– then a Black she would be.
Keeping Lily and his little Rose safe was unfortunately getting more and more difficult by the day: even with Sirius' help the war was getting ever more heated and even though Potter Manor where he'd grown up had excellent wards it was large and its location practically public knowledge. If the Death Eaters decided to besiege it they would take casualties but eventually break through. So rather than stay there James rotated his family through the half-dozen smaller cottages, holiday homes and town house the Potters owned on an irregular schedule while Lily threw herself into studying Charms and Runes in a quest to find a more permanent solution.
Lily's initial solution was runic and her Mastery project: using Rose's cot as a focus she built a protective ward to guard their daughter from harm. All three of them regularly donated blood to the runes to strengthen them and so long as Rose was in the cot when attacked nobody else would be able to remove her or harm her. Unfortunately the ward was unlikely to hold up against the Killing Curse or Fiendfyre, but that was a pretty small loophole really. Lily was still researching charms when Dumbledore suggested the Fidelius Charm, which he had found in his library.
James and Sirius both found it hilarious that, rather than let Dumbledore cast the charm, Lily insisted on learning it herself. She didn't manage to master it until after Rose's first birthday, by which point things were getting slightly desperate as four of the Potter residences and Sirius' flat had all been invaded and trashed by Death Eaters.
Dumbledore offered his own house to James and Lily then, stating that since he lived in Hogwarts and had for over fifty years now it would be a perfect place to go to ground. Lily agreed so James went along with it, intending to keep their hiding in Godric's Hollow as a short interval during which he could fix up the ruined properties and Lily could cast Fidelius on the still intact ones. It would also give him time to root out the spy in the Order, as for the Death Eaters to have found some of the destroyed properties a spy had to have told them: the only living people to have visited them were all in the Order. James didn't believe for a moment that Remus was the spy, but was happy to let others think it as it would encourage complacency and give him and Sirius a chance to catch the real traitor.
It was mid-October when the house in Godric's Hollow was finally ready to move into, so James, Lily and Rose settled in. But Sirius was worried about news of their using the charm having got out and suggested Peter be secret keeper while he acted as a decoy. Lily didn't like it but James could understand where his brother was coming from, so persuaded his wife to agree. It took a further week for them to track down Peter, but their old friend agreed at once and Lily cast the charm on October 26th.
Five days later James learned who the real traitor was and cursed Peter Pettigrew with his dying breath as Voldemort stepped over the threshold. His last thought was a plea for Lily's runework to hold up and for Sirius to look after their daughter.