She has the chance for more privacy, and she’s going to take it.

If you didn’t grow up having to share a bedroom into your teens, you have no idea what that experience is actually like. Sharing a bedroom with your little sister is pretty untraumatic when you’re five, and your sister is three, but as you age, it can be a real pain to basically never have a moment alone, even when you’re sleeping. Having your own bedroom is undoubtedly a privilege when you’re a child, so if you’re one of those kids who grew up not having to beg your brother to be quiet at 2 AM, you’re a lucky duck. 

There are certainly levels to sharing a room with siblings. It’s much easier to share a room when you’re only a couple of years apart from your sibling. If you’re a teenager sharing a room with kids who are still in Elementary school, it can make an already fraught sibling relationship even more challenging to manage. You can’t force siblings to be close just by putting them in the same bedroom.