why do parents show favoritism?

I'm the big sister. Cooking, cleaning, in school, protecting the others, trying to guide them in the right track. The boy doesn't do shit, stay in trouble, doesn't even take a shower with out being told & she still treat him like a king. No matter what he does. I get cursed out, slapped, called a bitch. He gets talked to. Wtf. It's so annoying. He's 15 not 5