I think abusive might be too harsh a word but it's definitely emotionally damaging to children for parents to have an obvious favorite. My husband knew his sisters were the favorites and even one was more favored than the other. It made him really sad.
I think most parents will admit to having atleast 1 favorite child. If they admit to it, it usually will be for specific reasons. Example being, my oldest sister used to be my parent's favorite, as she volunteered to always help out with everything (up to a point, where she started showing abusive behavior); then along came me, and the cat came out of the bag, and I was publicly listed as my parent's favorite.
It's certainly not right but sometimes it's easier to love one child over the other. I had two children, a son who made my life difficult for most of his life and a daughter who was a really easy child to raise. It's difficult not have have a favourite but I will say this. The minute I think something has happened to my son, and he has put himself in danger many times, I am in a terrible state of anxiety. I fret and worry until I know he is safe so even though he drives me crazy at times I know that deep down I love him.
If I had to give a straight answer to your question I would say I love them both but in different ways.