My daughter wants to have a sleepover -- the mother of the friend she's invited has informed me of her daughter displaying inappropriate sexual behavior. What do I do?

I'll preface this by saying that my child is 7 years old and in 2nd grade, so these are prepubescent children to which I'm referring.My daughter, we'll call her E has a friend we'll call B. B first came to my daughter's school last year, and their relationship started off rough. B bullied E, physically and emotionally. There were instances when both me and B's mother were called to the principal's office (separately) to discuss ongoing problems between the girls. On one such occassion B backed E into a corner, called her a b**ch, and hit her in the face. Another time B pushed E down on the playground when E refused to give her the bracelet she was wearing, and there were a few instances of the same ongoing behavior toward other kids as well. At one point B was on the verge of being kicked out of the school, as she has been from her previous one.E is a very loving and forgiving kind of kid -- it's one of the things I love most about her -- so over time the bullying stopped and E has left it in the past. This all happened last school year.Flash forward to the present. E and B have became "sort of" friends (that's what E calls it when we've talked about their relationship). Apparently B is temperamental and is prone to outbursts, and when she acts that way, E just leaves her alone and plays with her other friends. Over time, B's mom (we'll call her A) and I have spoken several times. I wouldn't call us friends, and our conversations have been strictly about the girls, but we have each other's phone numbers and she has added me on Facebook. During one of the several conversations we've had, A divulged some of B's background to help me understand why she sometimes acts the way she does. Apparently A is bipolar, and although it has not been officially diagnosed (because of B's age), A has been told by her physician that B has the same disorder. That brought a lot of B's behavior into perspective, and it makes sense. That's not the issue, as everyone is fighting their own battles, but it is relevant to the current dilemma.During the above mentioned conversation, A also told me that there were a few things that I "needed to watch out for should B ever come to our house." The outbursts were one thing, her sexual behavior was another. A went on to tell me that B had been caught several times masturbating. Not privately, but rather in plain view, like while sitting on the couch watching TV, or hiding behind a chair in the living room. I understand children are curious by nature, so I asked if it was more like her exploring her body, and she told me no, that B would masturbate "to completion" and when asked why (A was afraid B had been molested) B told her that she hadn't been, that she'd just "figured it out and it felt good." Not wanting B to feel ashamed for the action, A told her that what she was doing was natural, but that it was something you did in private away from other people.The situation was resolved, and there were no further incidents until a few months down the road. A walked in on B touching her younger sister (who was less than a 1 year old at the time) beneath her diaper which led to a conversation as to why it was wrong. B told A that she just wanted her sister to "feel good like she did when she touched herself." Supposedly they worked things out and there haven't been anymore situations like that as far as she knows, but that's not to say that there won't be in the future.I was molested by an older cousin when I was young, so this is a very sensitive topic for me. I've always done everything in my power to make sure my children were not put in situations like that, but I feel by allowing B into my home I'm putting not only E at risk, but also her brothers and sisters as well.I don't know what to do. The obvious answer is to monitor them closely, but with 5 kids in the house (the number of children my husband and I have) that's not always feasible. Kids can get out of your sight easily, and not only that, but these aren't babies we're talking about... They're going to want to jump on the trampoline outside or play in the bedroom with toys.My daughter has asked repeatedly for B to come over, but the possibility of things going wrong terrifies me so I've made excuses and put it off. That's only going to last so long though... What do I do here? via /r/Parenting https://ift.tt/2o58Imh