It means she treats one child like they are perfect (and have high expectations for them) but treat one of the other children as if they can do nothing right. Then there might be children who are neither.

For my sisblings and I think means our grandma tried to take me out to do special things (zoo, walks, etc) and telling me that I was wonderful for doing age appropriate achievements but was nasty to my sister (until my brother was born, and then she treated my sister ok but tried to be nasty to my brother). My mom shut that down really quickly as soon as there was a scapegoat (golden child treatment is harder to recognize as different from normal grangdparent spoiling with a total, but she shut that down too).