C.O.
Mom:
As long as you make it an issue - it will be an issue.
IF you are secure in your relationship and an adult - then I would not give this "little girl" a second thought. However, you have said that you always have to initiate sex with him.
If she is promiscuous - what are you going to do about it? Seriously - you can't control her or her actions. You can only control yours.
If he isn't saying anything to her - he might like the attention. He might have pursued her. You cannot tell him to tell her to shove it...you cannot fore anyone to do anything...you can lead a horse to water but you can't make it drink..