Given the last you have just written, I wouldn't go, or maybe just for the meal, but I personally wouldn't go and subject myself to SIL.
I have similar issues with my SIL. When RAH was in rehab and I was seriously considering divorce (really because of how MIL had my head twisted) she sent me the most hateful text that my RAH had been a happy person when he met me and that I was an unhappy person and never would be happy and she was glad none of them would have to talk to me anymore and pretend to like me. Since then she is not a part of my life. My MIL is not part of my life either. I don't prevent RAH having a relationship either. I just choose not to be around them. It sucks at times because I miss out on my niece and step nieces' lives and I love those girls to pieces. It was my nieces 1 year bday party today and I didn't go just because I cannot be around MIL or SIL anymore. I am considering volunteering to work Thanksgiving and Christmas because my FOO is pretty dysfunctional too.
I would skip it. It sucks to miss out on stuff like that but is being around those people and the grief it will cause worth it?