Your daughter is fine but unless your son is disabled / has other potential issues (like autism, severe depression) you need to get him on the right track. No 25 year old should be able to get away with the excuse of being unable to "adult well enough to live on his own". Your son of course has responsibility but that's really your problem to fix if he won't
One thing having twins did for me was push me way over to the nature side of nature vs nurture. They're so different, and always have been. And I don't expect to have more control over their lives than my parents had over mine at that age.
What do you mean by seriously autistic? You said in another post he does tasks around the house and took care of your wife when she fell ill. A seriously autistic person would not be able to do that. A seriously autistic person would require constant adult supervision.
Are you sure he's not just under-socialized and addicted to porn and video games? How does he spend his days? He might simply be a western hikikomori.
I don't mean to pry, but I'm kinda curious. What do you mean by function well in society? Some people with down syndrome can still live a semi-normal life and be employed, are you saying your son is worse than that? Can he talk to people? Can he drive?
He still doesn't drive. He's resisted it (mostly passively) pretty hard. He even has his own car. But to him, it's an alien thing, something he doesn't want to do.
He can talk to people who share his interests at the moment, or in highly structured interactions (he works retail and is pretty good at it), but in free-form "polite conversation", he's basically paralyzed. I seriously worry about if he ever has to engage with the police, because I don't think he could obey their shouted orders.
I see, in that case his dependence at this stage in life is understandable. Still I hope he's able to overcome the difficulties being autistic presents and figure things out.
Maybe he would enjoy games like factorio or Shenzhen I/O, those games are pretty fun and are good introductions to learning skills very similar to programming
