Why do my in-laws openly favor some of their grandkids?

My husbands parents have 8 grandkids. 4 girls from their oldest son, and 2 girls 2 boys from my husband(their younger son). The oldest son has four girls from 2 wives, then my husband has 2 girls from previous marriage, and we have 2 boys. They favor the oldest granddaughter, and my oldest son( the first grandson after 6 granddaughters). At Christmas the oldest granddaughter(she is 20),and my son(he is 5), get twice the presents that any of the other kids get. And they do it right in front of all of them. This will be my youngest son's first Christmas so I guess we'll see what happens with him. Why do they think this is ok? How could they be so cruel? I am very close to my nieces, and my stepdaughters. I feel very guilty at family functions!!