Family problem please read below?

I'm a 16 year old and I am confused. My parents are separated. My dad cheated on my mom and then moved in with his lover and they now have a kid, my half brother. I have one brother and one sister. My sister has cancer. My mom is 40 years old and she's with a 28 year old man who I call my step-dad. My mom and step-dad recently bought a house together. My dad won custody over my brother and I but not my sister because my dad isn't capable of taking care of her. To get to school I take a bus, a train, and another bus. The total amount of the trip is around 30 minutes. My mom is a money loving person and my dad is an over protective parent. We're originally catholic but my mom is experimenting with religions. I have a boyfriend who is 3 years older than me and he's the best but we tend to argue alot. I want the fighting between me and him to stop but I can't because then I disrupt my family life. My mom hates my dad. If I spend time with my boyfriend or anybody else, my mom doesn't want me visiting my sister because she says I don't care. I really need some help, I need some advice on what to do. I'm stuck in the middle of everything and if I do anything, I disrupt the balance. Please any advice would mean the world to me, thank you