I don't agree with my sister's crazy baby drama
Is there something specific that upsets you about her?
She often tells me I'm a bad brother and blames me for not doing enough
It sounds like she is not taking responsibility for her choices and is blaming anyone she can besides herself. It's okay not to agree with her on this issue as she is just being selfish.
I have to stand up for my sister against you, since I'm the only one who cares about her and I know how important is it for her to vent out this aggression
I get that, but it is okay to disagree with your family at times. You are not always going to get along with one another.