"As I can't be baby," Tootles said, getting heavier and heavier, "do you think I could be a twin?"
"No, indeed," replied the twins; "it's awfully difficult to be a twin."
-J.M. Barrie; Peter Pan
For all the squabbling, neither of the twins is entirely certain of their birth order . . . or if they even have a birth order being magically wrought from some greater being's soul and reincarnated through time on top of that.
It starts out as a bit of a sore point for the both of them.
It has been 'suggested' that the twins just ask the Scarlet Witch. Teammates have 'offered' to decide for them. Letting it go entirely would work for pretty much everybody.
Tommy and Billy never do anything the easy way, so they come up with their own solution and it works for them. Most of the time.
During the day, Tommy is the oldest.
Aside from the occasional token protest, Billy lets him because it's a new experience to be younger, to be protected like this. It's him (Billy), and it's Tommy looking after him because his brother is brash, annoying, and overprotective in scarily simple ways.
The rest of the world doesn't get to mess with Billy's happily ever after. Not on Tommy's watch.
Tommy had been off throughout the whole crusade-the wit a half-second too late and stale, without the same bite as before-but Billy could understand that. The whole adventure kind of stomped on his twin's issues in new and terrible ways while Billy had been distracted by his own.
Even as guests/prisoners of the Avengers, Billy hadn't given any credence to Teddy's brief suspicion that the Avengers might have gotten to Tommy. The blonde was afraid that Tommy might be psyching himself up to take out Billy if necessary. Wasn't that the approved Avenger plan with Pietro and Wanda?
It hadn't exactly worked on the first generation of Maximoff twins.
Billy trusted his twin, but it had been a shock to find out Tommy thought the same about Teddy. Of course, betrayal was kind of an issue in their family, but the point is that Tommy had a Plan B: Get in there, grab Billy, and run.
History really does repeat.
It's actually been really great for Teddy and Tommy's friendship-the whole mutual suspicion thing, not the history thing-because once they realize they both fear the same exact thing, they join forces. It's not always exactly convenient for Billy, but they make a nice matching set of green bodyguards . . . and Billy kind of likes the color green.
They more or less share an apartment in college. Tommy helps them move in and somehow never leaves, which gives him and Teddy better opportunities to bond over Billy's safety. It also gives Billy his chance at claiming the senior sibling title.
At night, Billy is the older brother.
He's had a lifetime of practice, and Billy likes to think that he's a good one. There's really only one thing that Tommy needs to be protected from in the end: nightmares. Between the containment facility and the divorce, there's no end of material for dreams that Tommy won't admit having.
So it's really just a matter of timing.
If Billy gives Tommy even a minute to regroup, he'll have an antagonistic (humiliated) twin deflecting with everything he can. If Billy wakes Tommy, the speedster will disappear out of fear and then be too embarrassed to return before dawn.
No, Billy has to get there just before Tommy manages to wake himself up so that the Billy is the first thing that his twin sees. It's tricky, but the power of teleportation helps.
"Just a bad dream," Billy will tell him. "Go back to sleep."
And if he's timed it exactly right, Tommy will lay back down beside him. Billy can press a hand to the back of his brother's head, tucking Tommy securely under his chin.
Tommy goes back to sleep if Billy's there to keep watch, because if any of them come back for Tommy, Billy will bring them to their knees. He doesn't have to murder to bring them down for good or to cause their work to crumble or to keep his twin by his side.
Billy wants his brother safe, and that's the strongest spell that Billy's ever woven.
If they don't acknowledge it, no one has to know. They come up with their own solutions, and most of the time it works for them.
It's not a sore point anymore.