Does anyone have inlaws favouring other grandchildren?

My sister in law use to get upset because my mom would spoil my children and not her's. Only reason why is because my children did not get like her and my brothers did. My kids only know my mom and dad. Their fathers parents have nothing to do with my children at all. I know this upsets my highly. Mysister in laws family always spoiked their kids. My parents where not good enough to match what her parents gave. They had more money than my parents. Their kids use to rub it in my children faces and my mom would interfer and give them the things they wanted.