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.