That POOR girl - the Step-daughter not the mother
I hate to say this but if I were you - I would stay out of it - people tend to shoot the messinger. It sounds like the guy NEEDS to find someone better than her to love his daughter...but then the son would suffer for that. So that's a really tough one to try to offer any help with....
You sure are in a hard place... but if it were a FRIEND of mine - I would probably sit her down.....with a Non-friend ...I would not get involved.
I sure hope she's not abusing that poor girl ...actually she probably is - Emotional abuse can be just as devestating as physical.... and no child should ever go thru either