Shared kids bedrooms and touching

I'll start of by saying we raise all our kids in gender neutral homes so so does my sister who has three kids. Which is who this is about.

Her two girls are 6 and 9 and share a bedroom. The boy is 10 and has his own smaller room. They are great kids and get along very well. The 8 year old wants to share a room with her brother out of the blue for more than just a night. Like actually move her bed in there and everything._

So the deal is my sister and her hubby are open about sexuality and anatomy. So they know what penis and vaginas are and what's the difference between them. They use correct terminology and don't make a big deal about gender. My sis and her hubs doesn't really have a issue with them changing in the same room which they already do if they need to._

Our mom has a huge problem with it even though they aren't her kids but the issue my sister brought up with me today was that she and her hubby are afraid they might start experimenting. Which they are at the age where they explore bodies a lot. They have suspicions that this is actually why she wants to switch rooms.

My nephew is not showing those signs and threw a fit when he was told she wanted to move in. Which they ended up not letting her mostly because his room doesn't have the space anyway. They didn't confront her about the times they've caught her trying to convince him to take off his clothes for games or trying to touch him and stuff trying to make it into a game. They're open about sexuality but I agree that incestuous touching can scar someone for life besides just being wrong. I have baby sat them many times and the girl is sneaky. She's gotten into my bed so many times after I told her to sleep in her own bed, and I'm a light sleeper. I wouldn't put it past her to use something like that as a opportunity to touch if she is really after that.

What would you do in this situation IF his room was big enough?