I was the daughter in that same situation. My step-brother was horrible and got away with everything. I came to resent my step-mother for all he got away with. If I ever said anything I was trying to break up the family. My dad and step-mother apologised years later but it was too late. I now hate my step-mother and waiting for the day she dies (she is almost 89).

I really hope things work out better in your situation.