Inequity between wives and children

Q: I am a religious young Muslim man. I treat my mother well, Praise be to Allah. My father is married to a second wife and he loves her and her children more than us. He gives preference to his children over me and my two sisters. My stepmother is an envious woman. She lives on the ground floor and we live on the first floor of the same house. My father sleeps, eats with the second wife and buys them food but does not do the same for us under the pretence that I receive a perpetual annuity. It is worth mentioning that his other children also receive a perpetual annuity. (Part No. 19; Page No. 195) He has not slept or eaten with us for approximately 13 years. When I see him, I greet and respect him but I do not go to visit him on the ground floor because I do not feel comfortable with my stepmother as she looks at me with her eyes full of hatred. When quarrels erupt between my mother and my stepmother, my father takes the side of my stepmother because he fears her. He does not respect my mother and he quarrels with her due to minor things even though my mother respects him. It should be noted that my mother served his mother for twenty years until the grandmother passed away. The question now is: Is there any sin or harm on me if I speak evil about my father in his absence because of the injustice we suffer from him? Is there any sin on my father because of his unfair treatment to us? It is worth mentioning that he barely gives my mother sufficient money for expenses. Is there any sin on me if I do not go to congratulate him on the `Eid Days bearing in mind that he does not come to us on `Eid to congratulate us? What is the Islamic way which my mother should follow to take her living expenses from him? How should I treat him according to Islam? It should be taken into consideration that my father comes to see us every two or three days for only 10 minutes a time.