I hear you on this one, man. I get to feeling the same way when I go over to my parents' house on "duty days" such as Mother's Day, etc. I dread going. My parents don't know about the abuse from my brother, either, although they do know that he had an affair with my first wife. And the do know that I'm not on good terms the last few years with him, so luckily they don't raise his name around me. I think they want to somehow "normalize" our relationship by pretending he doesn't exist around me.
I'm glad for your and my sake that we onlly have to make these kind of visits a few times of year. I just try to focus on the people who really DO mean a lot in my life during these times. It makes having to deal with the toxic people easier.