Caring for an unloving parent

I was raised by an insensitive, unloving cruel, selfish man.  Intentionally cruel and enjoyed it.

He has caused me much heart ache by not giving me the love and care that I so much deserved.  I am sick and fatigued.

He is old and sick and has had a good life and now I find myself feeling compassion for him,  He still treats me disrespectfully, but I still help him because my brother who he has given everything to(because he is a boy ,he is cruel and selfish also) does not help at all, he just takes money from my dad.
I do not get or ask for anything.  My question to you is that I feel that I should do this but I do not know why because I do not enjoy being with him because he is still not nice.  I just feel sorry for him.
 Am I doing the right thing, or am I harming myself by being around this negative person who makes me feel inferior and not loved, but used?
 I trust your spiritual intuition and I hope that you will answer this question for me.
 Will this help, or damage my spiritual growth?