>>2118
Some departures from reality are a given, and suspension of disbelief is hopeful at best in a story like this, but let's see if I can explain some of my reasoning.

The lack of reaction was, indeed, because their parents knew that nothing particularly bad could happen; that, combined with several years of gradually escalating violence that's gotten them well and truly used to their childrens' antics. It might be less clear why Alyssa's father was so insistent on the seat belt. In my reading, there are three reasons: He got into the habit earlier in life, he doesn't want splattered bits of Alyssa *all over the car* in case of an accident, and there actually is a threshold beyond which she'd be (temporarily!) dead.

To be clear, however... I don't know what OP's intentions were, but at least with my reading of things, the healing factor is technological rather than magical. Which is why I'm "Doctor Faust" rather than "Mephistopheles" right at the moment. They've been thoroughly re-engineered; their bodies have backups for everything, and can indeed recover from some quite impressive trauma, but as I mentioned there's a limit. Beyond that limit, she'd die. And be reloaded from backup, in the unlikely event that they can't scrape up enough DNA to get an up-to-date copy of her mind. The fact that they can't die *permanently* doesn't matter to the twins, because any scenario in which their parents get involved is an automatic loss. I hinted at the fact that it's an SF world with my references to "lifters" and "auto-cars"; background details of their lives that the two of course would never bother to seriously wonder about.

It would be entirely possible for Alyssa to speak to her father, explain what happened and get her body fixed, but that would be far too embarrassing a conversation for her to want to have. Which, of course, Alex knows. They know each other quite well, and despite appearances they don't actually hate each other; it's more the usual sibling rivalry, amplified by living in a world where these fight don't have any serious or even long-term consequences. The "maximum pain" Alex referred to is about the level of a papercut; any damage beyond that always feels about the same. It's enough to to avoid the sort of injuries you get if you can't feel pain at all, but it avoids any real suffering, and if there hadn't been such a limit they would never consider hurting each other.

So, over to your two main concerns, the mechanics and Alex' motivation.

For the latter, two possibilities. One, he knew it'd embarrass his sister more than him—because he was prepared, and had a convenient bulky sweater to hide his new figure with—and also because he doesn't always think things through very well; he'd focused mostly on *her* experience, without thinking much about how it'd be for himself. Two, it's possible that they were *already switched*, something I hinted at a couple of times. Alex has grown up as a boy, but he wasn't necessarily born that way. Of course, there's no way to confirm this right at the moment, and he could easily be wrong about that (vague, unformed) suspicion.

For the former... yeah, they're fragile. Some of that is simply necessary for the story to work, but my in-story explanation is simply that, although they heal up to "normal functionality" pretty quickly, it actually takes days to weeks for their bodies to reform the sort of strong bonds that prevent made-from-plasticine syndrome. Alyssa's ribs, spine, organs... everything broke apart pretty easily, because she'd already been broken apart in the same way relatively recently, and there's nothing preventing them from using significantly more force in their normal fights.