Is it inappropriate to not attend a family gathering because of my sister's fiance?

My sister is recently engaged to a man who none of my family likes. He is loud, rude, and talks about things that are completely inappropriate for mixed company. (Eg: Talking about his brothers girlfriends "orifices".) He treats my 7 year old terrible, even calling him stupid at one point. Him and I have... show more My sister is recently engaged to a man who none of my family likes. He is loud, rude, and talks about things that are completely inappropriate for mixed company. (Eg: Talking about his brothers girlfriends "orifices".) He treats my 7 year old terrible, even calling him stupid at one point. Him and I have gotten in to spats before but I try to avoid him. My parents and sister all see that there is a problem but none are willing to stand up to him. My sister is coming for Christmas this year and her fiance is coming with. Would it be rude of me to not attend to avoid confrontation with him? My mom is constantly saying I'd better get used to it because he is here to stay. I think i shouldn't have to have him in my life if I don't want to.
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