I am in a similar situation. My younger sister thinks the church is “mostly bull$#!t” and is getting married in October at a botanical garden (to a non-Catholic.) She was baptized and confirmed, so I know her marriage will be invalid. I’m really torn as whether to attend or not. She knows how I feel and that I am sad for her choice, but I love her dearly (she’s my only sister and we’re only 28 months apart - we’ve always been close) and I know that by not attending, I would break my parents’ heart and cause a lot of grief in our family. My parents also wish she’d marry in the church, but they said there is no way they aren’t going to attend their daughter’s wedding.
Then, I have the whole issue about letting my kids attend and be in the wedding! This is a hard spot to be in because you are being pulled in so many directions. I feel for you. I can’t say what to do, but I am leaning toward writing my sister a letter explaining that I will attend the wedding out of love for her and to keep peace in the family, but also charitably laying out my concerns and objections and letting her know that I will pray for her and her husband. She thinks I’m being dramatic and cultish. I think she’s being immature and irresponsible. But at the end of the day, we’re sisters and we love each other. It’s hard.