My sister in law basically lives off of my mother in law and her kids get everything. My mother in law rarely gives our kids anything because we have the money to do it ourselves - and I get a bit miffed at times.
I know that feeling too, at least somewhat. Seems like clear favoritism to me, but maybe I'm wrong.
She always says "If I didn't buy it for them, they wouldn't have it".  But it's a bit ridiculous because my sister in law DOES have the money, she just refuses to raise her own children.
That type of stuff always makes me so angry, since it's so clearly favoritism and hypocrisy and they just don't care one bit.