I know quite a lot about step mothers and dads who will take their wives word over believing their own kids . My dad was married 7 times during his lifetime and 5 of those 7 times happened before I was 15 years old. All 4 step mothers came with kids of their own .
Trying to 'fight' to win in a head to head with a step mother that has your dad wrapped, is like trying to move a mountain with a teaspoon. It can't be done the cards are stacked against you.
Now trying to keep your sanity until you can legally leave , that's a little easier. When she comes at you trying to pick a fight let the words " I didn't marry her , dad did ' go around and around in your head until they drown out her BS. For one, it's true, and two it puts the responsibility for her happiness right back where it belongs on in her lap . And three, since your busy reminding your self that she isn't your responsibility you won't be playing into her picking a fight.
You said something about your dad being sick and her and her child molesting drunkard son are taking advantage of your dad , it sounds like her and her son are doing all they can to make sure you, as his legal heir, won't have anything to inherit. There is nothing you can do about that simply because your dad and her are married.
Now about her child molesting drunkard son , the only thing worse then a child molester is one who never gets exposed as such. You need to tell someone else about his molesting you , some one who will listen to you and have him investigated .