What a very worrying situation for you!!
I am very sorry that you are going through all this.
Would it be possible to have two separate celebrations for your little girl~ with the one for your Mum and your sister a bit later and more low key ~ like a children's meal at a take-away, or something? That way, the families would stay apart, but all could be involved. And you wouldn't have the worry of secrets coming out.
I agree with Pudgie's Mom about showing your sister your concerns, printed out, rather than simply confronting her, as if you know best. Fifteen-year-olds will always respond badly to that ~ well, many older people might too
I can certainly sympathise with your worries, especially as this girl is so young ~ not much more than a child. There is every chance that he could be taking advantage of her. Maybe they are taking advantage of your mother, too.
Of course, they might be nice decent people in a difficult situation, but you cannot know that and your sister is too young to be taking such risks ~ in my opinion
