My grandparents on my Dad's side treated all of us differently from their other grandchildren. At Christmas, everyone would get $5, but my cousins would get $5 and an envelope "to open at home."
My mother preferred the dog to me. My sister was and is her favourite, blowing all theories about the middle child, which she is. My mother even treats my child differently to my sister's.
Right before my grandmother died, she told my mother that they treated her, my Dad, me and my siblings badly because they were jealous of us. She apologized to my Mom for being so petty. It was nice to know why, but we still can't make good memories where there were none.