How do we get him to change back?

I know that if people don want to change they won't but I can't just sit here and mope. My mom and dad have had a steady 18 years of marrige with a few arguments here and there, but nothing big. For the past four years my dad has been puting his friends infront of my mother, my two younger brothers , younger sister, and I. My mother is from Cali. and my dad is from houston texas. They met through work in claifornia and dated for three years. They were going to go back to texas to get married when my mom threatened to call it off because he was putting her last on his list of prioritise. He promised he would change if she married him. He did change until four year ago. When the johnsons moved in next to us my dad and mr j became good friends while mrs j acted as petty as a teenager towards every one. Their son taylor is a comlpete *** and every time he and my brothers get in a fght he backs up taylor and not his own kids. Taylor has gone as far as telling my 2nd grade brother and my 4th grad sister how to have kids, the whole shabang and when my dad found out my mom was gonna call his mom about it he wouldn't let her because he didn't want to make them upset with him. Then when my mom trys to disipline us he won't let her. She tries to tell him about what he does but he won't even respond. Its like he doesn't hear her. She wants to threaten divorce just to try and shock him into listening but she's afraid he really will divorce her. The only reason she hasn't yet is because her job and my job won't pay enough for my three siblings. I know its not my business getting into my parents relationship like this but I hate seeing my mom miserable. Please I don't know how to help her and my younger siblings are noticing how bad their relatonship is.