Is it normal to let people change your mood just by showing up?

My boyfriend and I have been together for almost 5yrs. We live together. He has 2 kids 18&21yo. I have 2 as well 18&17yo. We all did live together at one time. His 18yo And mine are both girls 8 days apart. They couldn't get along so my daughter moved next door to my moms. I am an only child. He has a brother that his mom doesn't acknowledge but gave birth to.
The issue is, his mm was always fake to me. Only when I caught him cheating did I realize how much. She actually refers to me as miss piggy. His 21yo son has stolen from us so much and abuses drugs and alcohol, has assaulted my boyfriend, his daughter and sexually tried to assault my daughter. His daughter once again lives with us and I have caught her engaging in conversation with her grandmother and referring to me also as a pig. Just yesterday she told him I saw her ex and was trying to get them back together ( my bf hates his daughter's ex) I did see the boy at a public eating place. The boy told me that he and the girl were secretly seeing each other. I did nit tell my bf, he never believes his innocent daughter would do anything sneaky. So when she and he ganged up on me I let it out and then some, pictures for proof. So today, nothing is wrong and his son who just was in court yesterday for hitting the daughter is here. The son stole from my son the night he aassaulted his sister. I don't understand why I cant walk away. The kicker is: the bf has tried to contact the girl he cheated with , he stays at his moms for days and never contacts me. I pay all the bills. His mom told him he deserves it cause he worked full-time while I was is it normal nursing school. He went to school and was doing good until his mom said she needed him to watch her dogs. He never went back. I worked as much as possible and put all my money (pell, child support and paychecks) into the house while I was in school.
Whats wrong with me? Im fairly attractive. My 18yo is off at school and my 17yo works full-time. His 21 yo steals does drug's and whatever he wants. His 18yo quit school works 2 hrs a day waiting tables at pizza inn and sneaks around. She also wrecked her car. Lied said it was a dog. I found out she had been drinking and doing drugs. My boyfriend believes his innocent daughter. His mom talks to the girl he cheated with and encourages him to talk to her. He will not stand up for me and I often catch him ssaying things t his mom that she wants o hear. I have told him to leave plenty. He never does. Help? That's only a fraction of what goes on. Help? I just want to run away. PS: he says he will marry me if I have sex with another man and hin and allow them to film it? I'm lost