My sister and I were very different; I was academic, she has other talents. I think my parents didn't want to praise me too much in case it upset her, but in our teens it was clear (to her at least) that I was the favourite. They sent us to different senior schools, with the idea that teachers would not compare her to me, but she saw it as not being 'good enough'. It has given her anxiety problems in adult life.
When we had two daughters we tried very hard to treat them equally, despite them having very different personalities. They have both achieved great careers, have happy memories of their childhood and try to give their children the same experiences they had, so we must have done something right!
My sister and I have become much closer recently; my daughters have always had a good relationship with each other. My only regret is that I didn't take more photos of my second daughter as a baby as she complains that we took loads of her sister but not her, and the ones we took all had her older sister in!