All you can do is go with your own gut feeling about it, you can make sure they are not left alone , it has to be a decision you alone can make,as the child was 5 year old when it happened it may indeed have been a one off situation, but who is to know , have you asked the doctor what he thinks about it ?
He says I am over-reacting and there is no way his son would ever do anything like that again---I don't know---like I said, my safety of my daughter comes first... He did not even believe his son was abused until someone told him recently it was abuse --- because he did not have the mind set yet to tell the girl no...
well then you have to find another Bf as I dont see how the concern will help you be happy or look after his 7 year old,if its only summer maybe it should be day visits and not overnights .What does your BF say about your concerns is he also worried ?
My concern would be in the evenings---when we are sleeping---I don't think I could ever sleep worrying about my daughter and her safety. :/
As you have this information up front, so to speak, it may be a good idea to make sure someone is always around and not let them alone you are for warned, Children of the same age often are curious about their bodies,I doubt that just goes away .you are aware so make sure she has someone around . .