My Mom and Stepdad have never been happy and I want to help my Mom

For context my mom met my stepdad when I was around 6-7 years old and I’m currently 16 my mom turned 19 the day she had me and was about 25 at the time i assume. They met on a dating site and moved in within about 3 months of knowing each other. My dad and mom got back together about 3 times after their divorce and he had cheated on her which led to the divorce. After meeting my stepdad we moved a lot and their relationship started off untruthfully I believe he never mentioned his 1 year in prison or that him and all his sisters were addicts like heroin and meth addicts. When I was 7 they had a boy which left my mom with me, my brother, and my new brother. When I was about 8-9 they got married and my mom got pregnant again. He had told my mom he was infertile when they first met and it turns out his body was a bit messed up because he was constantly smoking marijuana, i have no issue with this, but he is very dependent on it to be in a good mood. We moved again but this time a few states away where he tried to have my mom committed until he found out his family in that state couldn’t do it because they weren’t blood relatives and i wasn’t able to see my dad because of distance. we moved again back to our home state and we had to move in with my grandma above her garage me, my mom, and my 3 younger siblings where my stepdad wasn’t allowed to visit because my grandma said he sent her a death threat in the mail when we were out of state which i strongly believe because he was constantly relapsing. My mom never did drugs or drank she was a smoker and that’s it there was no reason besides mental issues for her to believe we’d be taken, but my stepdad constantly threatened to and she was scared. We moved out of there and moved to a house in another town and my stepdad worked out of town and was our only income we had no car and walked everywhere while it was winter. He was cheating on my mom while gone and probably had been for a while and his sisters knew. one of his 5 sisters and her three kids and husband lived with us for a little bit and they were drug users. They eventually left and my mom was worried my stepdad would hurt her and her dad came and stayed with us for a while since he was at the time clean. We moved again to our current home when i was 11 and it was all fine still fighting everyday like there always has been and about over a year ago he relapsed and cheated on my mom and there’s always been threats of divorce but he knows shed never do it. My mom has confided in me for the past several years and told me everything and it has all made sense looking back. I never liked my stepdad and everywhere we lived his sisters moved. they’ve lived with us three times in our current home and I’m always blamed when them and they’re children have done anything its been a year since anyone lived with us, but the fighting never stops. Today i found out i was sick and was trying to help calm my younger sister down while my stepdad brushed her hair and we’ve all been sick all week with the flu and I’m supposed to be out till Monday but he got upset and said were all going to school all because he was frustrated with my sister crying he hates hearing it he prefers to nap and play games. Him and my mom started fighting and he said she could lay the divorce papers down and hed sign them but i know he doesn’t believe she will. I just wish shed leave him shes not nearly as scared as she used to be and we’re surrounded by her family that’s supportive and hates my stepdad too, but she works about an hour and thirty minutes away and we have school who would pick my siblings up ,my brother is autistic and the bus isn’t an option and how would we afford rent or phone bills or any of that without the extra money she makes far more than him because he does under the table job and is hardly ever paid but we need the money in a house hold of 6 people. He’s a felon and doesn’t have many job options besides under the table because he refuses to work an actual job like retail or fast food because he cant listen to authority or hes embarrassed. I want them to just divorce I want to go live with my dad. How can i help her