I dont really know where to begin actually. 

I (f21, german) know my stepfather since probably 2007. My mom married him around 12 years ago. They bought a house where i lived until i was 17 years old, then i moved out. My 18 year old brother is still living with them. 

My stepdad lets call him "Sven" always was a very angry person. No matter what we did was wrong in his eyes would made him totally explode. It started from drinking out of the water tab and dont use a cup, letting a charger in the Socket or filling up the ice thingy in the freezer with another tasting liquid. He punched holes in the door and said horrible things to us. He has two freinds but just sees them at work and otherwise he just sits in his own room. He has several Hobbys that come an go in a weeky rythm, he buys things look at them for a week and then sells them and get into something new and never sticks to anything really. I dont think ive ever saw him laughing from heart once.

As i grew older i noticed my moms alcohol problem.
Turned out she used to be a alcohilc since i was 5 years old. The Situation in my household was unbearable for me so i took the first Chance to move out that i got. 
Because of that i always thought my mom was his main problem. That he acts out like that because she was drunk constantly and her emotions were all over the place leaving her laughing and then crying in a matter of two minutes. 

I confronted her and she went to drug rehab and made an amazing recovery. She is dry for almost two years now and we have a wonderful relationship since then.

Right after rehab Sven was like another person too, he havent exploded at all, no fights, no unnecessary arguments. But around 5 weeks after he changed back completely. It was like nothing ever happened. 

I think it is because the first euphoria went away and, what i think is his depression, came back. 

If sven would be a person you can talk to like a normal human being i would but i really need advice. 

He is very very conservative, dont like gays, other races, dont even try to talk over polotics, he dont belive in things like calories and that sugar is bad for your health even when he got diagnosed with gout and would really benefit from adjusting his diet. 
But he never listens to anything, ever. 
My brother and i wanted to get into leopard gecko breeding and we told sven over and over for two years that we are going to do it when my bro is 18. As is was time sven completly escalated and told us to ask him first and we never said anything to him. 

My mom did so much and went through so much and he makes her miserable, she is stuck with him in this house that wants to be payed and i fear her to relapse at one point or another. 

I need to talk to sven to get professional help for him. 
But i am afraid that he will not listen like he always does. 

We all saw in that short period of time that it can be better but i dont know what to do. 

Sorry for that monstrous text but i am desperate