Here's an alternate version of the "Fred and George find out about the Marauders" scene. I couldn't quite work this one into the Weasley Family Meeting, but here it is. Setting: Ron has already explained about Sirius' innocence, and after the meeting he pulls Fred and George aside. They ask him how he can claim that James, Sirius, Remus, and Peter were bigger pranksters than themselves.
"Well, it's funny. They had these odd nicknames for each other, based on the animals they could turn into. Hm, I believe they nicknamed Peter for his rat's tail, James for his antlers, Sirius for his doggy feet, and Lupin for the full moon. If I could just remember what those nicknames were…" Fred and George's eyes bugged out and they stared at Ron, then at each other, and gasped with sudden realization.
"He can't be saying what I
think he's saying."
They looked back at Ron. "Can you?"
Ron smirked and nodded his head.
"Professor Lupin is
"Sirius Black is Padfoot?"
"Peter Pettigrew is--"
"--SCABBERS is Wormtail?"
"James Potter was Prongs?"
Ron was still grinning. He was glad he got to be the one to spring this information on his brothers. Then Molly called them all for lunch, and the twins were babbling the whole time about how they had Moony as a teacher for a year and never knew he was Moony.
"Who's this Moony, dear?" asked Molly. "I hope you're not making fun of Professor Lockhart." She blushed.
"No mum, we're talking about Professor Lupin. Ack! I can't believe we had him for a year and never got to ask him about his years as one of Hogwart's greatest pranksters."
Molly raised her eyebrows. "Well, you'll just have to ask him when we get to Headquarters."
"Yes!" Ron grinned, "I was hoping he'd be there. So he's part of the Order too?"
"Definitely," said Arthur.
If you like this stuff, check out my C2, "Magical Mischief Makers", wherein I have collected all the stories I could find on ffnet in which Fred and George either meet or find out the truth about the Marauders.
And now for the final bit of this story...
"Professor Lupin! Professor Lupin!" Fred and George ran screaming toward their former teacher, who looked up bemusedly from his cup of tea.
"Hello, boys. Good to see you again." The twins stopped two feet away from the older man and stared at him for a few second as if they had been hypnotized.
"Er, are you two all right?" Remus set down his teacup and took both boys by the elbows. "You haven't been eating anything funny have you?" He sat them down at the table across from him.
The twins snapped out of their daze. Then both started babbling at once, so quickly that it was impossible to distinguish which one of them was talking, so everything just ran together into one big sentence. "Sorry, Professor, it's just we've wanted to meet you for such a long time and now we finally know who you are and we just wanted to thank you for everything you've done for us and we owe you SO much, and Ron finally told us and I can't believe he never told us and Harry never told us and we're really glad it's you because you're the best professor we ever had and can we get your autograph?"
Remus furrowed his brow. They hadn't figured out that he was the author of Hairy Snout, Human Heart, had they?
"What's this all about, Fred? George?" The twins stopped mid-babble with their mouths open.
"You're Mr. Moony!" exclaimed George, at the same time Fred said, "You wrote the Marauder's Map!"
"Oh!" Remus sighed in relief. "I'm assuming you two are the ones who gave the map to Harry, then." He sat back down and picked up his tea.
"So can we have your autograph?"
"That map is the secret of our success!"
"Please tell us how you thought of it!"
"Well, certainly," Remus replied. "Marauding with the Marauders -- that was the best time of my life. But you can't let me take all the credit. I was only the one who did the tiresome research. James and Sirius did most of the spellwork. Hey, Padfoot!"
"The twins' heads snapped so quickly to look at the kitchen door that they winced and massaged the back of their necks. Their eyes grew wide and took on the hypnotized glaze once more. "Padfoot," they whispered in reverence.
Sirius Black was rather confused as he came into the kitchen to rummage for something to eat and found two identical redheads staring at him as if he were a two-headed hippogriff. "Uh, what's going on? You two are Weasleys, right? Er, are you okay? Moony?"
Remus chuckled. "Well, apparently these two are fans of ours. They're the ones who gave the Marauder's Map to Harry, and I, for one, would very much like to hear how they got it, since I'm not their Professor anymore and I don't have to feel guilty about encouraging mischief-making."
Sirius let out a barking laugh. "Fans of the Marauders, are you? Well, I can't say we didn't have plenty of fans when we were at school! So tell us where you got our map, then."
Fred and George had recovered from their worshipful trance, and identical evil grins appeared on their faces.
"Well… when we were in our first year -- young, carefree and innocent--"
"--well, more innocent than we are now -- we got into a spot
of bother with Filch."
"We let off a Dungbomb in the corridor and it upset him for some reason--"
"So he hauled us off to his office and started threatening us with the usual--"
"--and we couldn't help noticing a drawer in one of his filing cabinets marked Confiscated and Highly Dangerous."