ESH- don’t go to the wedding. I understand where you’re coming from. That being said, a wedding is not the right time to wage this war. Your sister phrased it in a horrible way. But she is also trying to build a relationship with her new in laws. I think it’s a more difficult situation than enabling homophobes. What if you have dinner with your partner, her fiance, and her new in laws either awhile before or after the wedding. If you have it before, maybe you can test the waters. If you have it afterwards, you can still work on that homophobia without taking the attention off of your sister.
As with every wedding post I see on here, give them this day. Give them a break. Don’t take the attention from the couple, even if it’s for a good reason.