10 years ago we went to my parents house for Christmas. DS was 11, DD was 4 and DN (nephew) was 9.
My DN literally had a pile 3 times the size of my two kids put together! These were just gifts from my parents, as his gifts from my sister would be given the next day.
My children opened their gifts, thanking the giver (or Santa) after each one. My DN opened his and threw each gift over his shoulder or pushed it away with a grunt and reached for the next one. My sister, mother and father never said a word to him. Honestly, I wouldn't have believed it if I wasn't there. DH and I said nothing either, as we were not allowed to "discipline" DN.
That was the last Christmas we spent with any extended family. BTW both kids noticed the very uneven number of gifts though neither said anything until we were on our way home 2 days later. (DN's gifts were also far more expensive than those given to my kids. Compare game consoles to board games and coloring books.)
4 Christmasses ago Sis noticed a big difference between what her MIL got Loren and Loren's boy cousin. (Since ON would have been 13 and the kids were toddlers differences could be put down to ages).
Sis point blank told MIL and Betty either you treat all the kids equally as possible or you will not see my kids (ON was included in that). They have been better. I know the boy's Mom still sees ON, Loren, Brett as the Golden Children by default. (Golden Child is Betty but she doesn't have kids).
Honestly I think it is more the problem with her older boy. He has some type of issues going on behavior wise. He was in the Katrina evacuation*, his Mom has on going problems with depression, his father lacked basic parenting skills (I stopped him once when he flew in a temper because the boy was "ignoring him". I was afraid he was going to shake the toddler. Thing was it was very loud with a bunch of kids playing and music blaring. I didn't hear him call the boy and I was right next to the child). They took some parenting classes, and somethings improved. Still the boy can't handle frustration at all. I don't know if it is just bad training, if he is on the spectrum, or has some type of anxiety/depression issues.
*Some families are still struggling with emotional issues from the evacuation.