This is a hard one - as what her stepfather is doing could be a simply notice and turn away thing that many people do and it means nothing what so ever to him - but on the other hand it is making your daughter feel uncomfortable, therefore, it needs to be addressed...... and should be.

I personally would talk with your husband about it (in private) and see what he has to say...... he might not even be aware that he is doing this - as it almost seems second nature (unconscious) for men to notice a busty women, but some times that is all he is doing "noticing her" - heck I usually can't walk past a busty female with out looking myself..... then I move on.