This is really all the information you need to make a decision. I think your therapist is 100% right. Your older daughter is sexually abusing her little sister. It is your job to protect your younger daughter from sexual abuse. Otherwise your therapist is a mandated reporter and would need to call DFS to ensure your daughter's safety. This issue is not about your older girl having her own room. It is about protecting your younger daughter from sexual abuse. Plenty of people on the forum have written about how sexual abuse from a sibling has changed them for life-- it is extremely serious. Just because they are both girls does not make it any less serious. The entire point of separating boys and girls is to try to prevent the possibility of this kind of stuff from happening. But in your house, the problem is with the older girl instigating things with the younger girl-- not with your little girl and little boy. So you need to separate the children where it is actually a real problem!!