Hello S.. Unfortuantely, I do know all to much to much about favoritism. My grandmother did it to me and to my brother. Apparently, she would spend more time with him from the time of his birth and spend more money on him...I bascially disappeared when he was born. Boys were more of her forte she would explain to my mom and dad. Bull! I say!!!
There would be times that my grandmother would buy him expensive outfits and totally forget about me. And I was the eldest child. So my mom would take back whatever my grandmother purchased, and then split the money between us. So when we were infants or children, my mom would take the money and buy us both something of equal value.
As for treatment, I don't know what to tell you. My mom and dad used to have talks with them saying that if she couldn't treat us both equally, don't bother coming around. For awhle she would treat me ok, but then she started insulting me. I think that I preferred the silence. Even today, my grandmother ignores me, and I'm 33. To tell you the truth, it does bother me, but I deal with it. However, it was a terrible feeling when I was growing up. I never resented my brother though. It wasn't his fault that my grandmother favored him over me.
My inlaws try to play favortism with my two girls and I simply won't have it! I tell them, if you want to buy something for one girl, you better bring two or do not bring nothing at all. You see, they favor the first born and shower her with whatever they bring. Then our second born is left in the background with her hands empty. My inlaws have tried buying stuff for Mairlynn and claiming that they could not find another one for Maggie. I take it, return it, and buy two of something that I know that my girls will love. So, I take matters into my own hands.
As for them taking the boy and not the little girl, I would tell them they could have the boy the 1st week, and the girl the following week...no more playing favorites. Although they may favor your son, your daughter needs their love just as much as your son does. If they don't want to agree with this arrangement, then I would temporairly cut off the visits until they do agree.
Good LUCK!