Is it very common to have a brother and sister live together after they're adults?

If for more than a couple months, and absent a medical need for one of them, yes, it would be weird. More weird than living at home with your parents when you are in your forties.

It would indicate an inability to make one’s own decisions., and an inappropriate dependence.

Very common? No, of course not, or you would know a lot of people in that situation. But I can think of circumstances that would reasonably bring it about; for example, both being single and, for whatever reason, occupying the family home after the parents are deceased.

Or if I as an older person were, let’s say, widowed and in my own home, and my brother were unemployed, broke, and going to be evicted from his apartment, I might well let him move in with me and share housekeeping responsibilities.

What does “weird” really mean in this context? Unusual seems strong enough. I don’t think it’s up to you or me to judge someone else’s living situation unless we’re affected by it somehow.

It’s not common but it might suit them for a while. Most adults might like privacy and having another adult in the household would make that difficult. If one or both of the siblings had financial difficulty, then it might help them. I can envision someone saying their brother or sister needed a place temporarily so are staying with them for a while. The scenario sounds like they’re both single without children.

That’s the situation on Anne of Green Gables.

I don’t see it often, but I would not call it weird. I don’t have a brother, so I can’t speak from personal experience, but if I knew a brother and sister who lived together I would think it’s nice they get along well.

I would live with my sister in a second if I wasn’t married and I needed a roommate for financial reasons and we both wanted to live in the same town.

Even now I offer her to come live with me if she wants a break from NYC.

I have a friend that moved in with her brother to help take care of his child when the mother died. The child was a baby at the time and now is in college. The friend still lives there. It doesn’t seem weird to me. Maybe in other circumstances it would.

Yes, especially in this age of ultra high rents. No different than regular roommates…

I don’t know about very common but can see it in a few circumstances might be easier on living expenses ,a brother or sister as a room mate might be easier than a stranger.

Depends one what country and what economic conditions you’re looking at.