Selfish Brother and Sister in Law

My dad is frail and can't do much for himself, he broke his hip around 10 years ago. I don't resent helping my dad but I do resent my sister in law being so rude and impatient with him when he repeats himself, she is a very critical person and I wish my brother had never met her. She snaps at me if she feels I am doing something wrong. They go on vacation and I work full time and I would sometimes like to go away for a week or two but I can't and they don't care or understand.

I think the problem is her, she treats my brother like he is a nothing and wears the pants in their relationship. I don't even like to talk to them because I never know when I will be snapped at or when my dad will get criticized by them.

My dad doesn't deserve this kind of treatment. I am glad I don't see them often. When they get elderly just wait to see who helps them out, it certainly won't be me.