Why does his mother treat my daughter different?

Hi, My Fiance and I met when i was 7 weeks pregnant, the man i was with turned out to be a complete , and turns out he was a drug addict. So I just got rid of him. So I met this really amazing guy, which i Knew he was the one! so we started dateing, and i met his family, to me they were different but nice. Anyways he sister was already pregnant too, she was 2 month farther long then i was. Well his mom always told me that she would treat our kids the same, because she would be just like her own granddaughter, and his sister was having a boy. So anyways when i had my daughter (her father was melada, so she wasn't dark but she had dark brown eyes, and like tanned skin) they kind of seemed to not treat her the same at all. like they just seemed to love the little boy more then my daughter. Now my daughter is 19 months old and the little boy is 21 months old. and everytime i bring her out there, she'll be playing with a toy and he'll come over and scream until he gets it, and then it upsets my daughter because he takes her toy, but yet he does not get punished, but when my daughter does something mean to him she goes on time out! and they are always buying him new stuff and never her, and they buy stuff for my fiances daughter which is 8 who neither he or his family have ever met who lives like in another province completley, but yet they don't even acknowldge my daughter unless someones around they'll do it for show, and it makes me feel like my fiance and i are the only ones who maybe care for her, why is his mother and sister doing this to my daughter. she just wants the same love as everyone else. please tell me what to do, because in there eyes they are bothering , but they are not... they actually make me just wanna cry sometimes, the way they treat that snottie, spoiled rotton little boy of hers, and yet my daughter always trys to get huggs and stuff and they dont even acknowledge her my fiance has got to tell them to pick her up and give her a hug.. why are they doing this?? and what should i do??