I’m sure you’ll get many of responses saying not to go, but I never understood that. I have friends from all types of religions. I have been to different weddings. I go because I am very TRULY happy for them and WANT them to be happy. Some people’s religious attention ebbs and flows and some will always be different from our own.
If you are truly against their marriage and would be willing to stand up and say that, then don’t go. If you want your sister to be happy and think the wedding is a good thing, then go by all means.
Sometimes a marriage will start of one way, but end up in another. Our in-laws wouldn’t be caught dead in a church before they were married in a civil ceremony. As they grew together as a couple and contemplated children, their views changed and they were pulled back into their faith. It was a wonderful transformation - like a butterfly’s life!