T.S.
Wow, that sucks :-(
By BFF's inlaws are like this. They give lavish gifts and tons of attention to her son, and practically ignore her daughter! These are their only grandchildren, and they have plenty of money, so it's a clear case of favoritism. Once her daughter got old enough to start noticing (I think it was the year they gave her little brother one of those big electric riding cars and they gave her a small stuffed animal---I am NOT kidding!) my BFF and her husband decided to tactfully bring it up with them. They were not accusatory or anything, they just wanted them to know how it made their daughter feel.
Well I'm sorry to say, it did not go over well. Lots of yelling followed by the cold shoulder which is still very cool all these years later. And they STILL openly favor their grandson!
Sorry, I'm not trying to be negative, I just wanted to share my limited experience with this kind of blatant unfairness. If I were you I think I'd try to let it go, as hard as that can be.