AnneMarie, you are quite right, the age and mixed gender sharing a bedroom is inappropriate. It IS your business because it is your child and part of your responsibility as a parent is to safeguard your child.
She may feel embarassed or uncomfortable and it is not fair that she is being put in that position. Tell your ex he is incorrect and anything to do with your daughter is your business.
Here is one on my orders that I fought for (Abbey)
I have two boys my ex's gf has two girls. They were living in a two bedroom house and I refused to take my kids to their Dad's because I have an order set in place that my boys are to have a room to themselves. Not to be shared with gf's kids. I think it is WRONG for your ex to even think that it is okay to put the two together. It's very much your business!!!