Is it normal to feel sad for my sister?

Growing up my younger sister got most of the attention between our parents, this would lead me to becoming incredibly jealous and bitter, and even hating my sister. I would bully her and hurt her until she cried or started hitting me, I didn't care about her at all.

I usually barely passed my grades at school while my sister was earning A's and B's. As I grew older I basically gave up, my sister would always outshine me and it was obvious she was going to become something in life unlike me.

I finished high school last year, and seeing my sister she is a completely different person. Her grades are bad, she has no friends, and she hardly talks to me or the rest of the family because she literally sits in her room all day. I've tried talking to her and being friendly but she wants nothing to do with me.

I found a letter laying in her room today, it was one of those future letters you write to yourself. Reading it I grew incredibly upset, she wrote down her aspirations to become a singer or actor and to have a husband and children, but the way things are now I just don't see that happening. I mean I know I fucked my life up and I'm nothing, but my sister was the one to be better and to someday live a happy life with a great job, she was the one that should be happy. I just don't understand. She's done nothing wrong and has turned out like this, she doesn't deserve it.

Why is life so cruel?