How about explaining the Church teaching to your SIL and lovingly saying why you aren’t going. And urging her to find out if that marriage was really null. If you attend, you do give scandal. You are celebrating concubinage. By the way, what gift do you give a concubine on their wedding day?
And if you attend, then people who hate the church will use you as an excuse for seemingly to excuse sin. “Her brother and SIL attended. So it must be okay.” You can’t win with that crowd. They will use whatever you do as an excuse to bash Catholicism.
My experience is that the way a couple tries to dodge and weave the rules and regulations preceding their marriage achieves its full bloom once the ceremony is over. I know of one woman who converted to Greek Orthodoxy because she was not willing to have an unconsummated marriage nullified. (Physical issues with groom.) And her new husband had been previously married and he balked at having his Christian marriage annulled. So they got married in a lovely Orthodox ceremony by a church that didn’t demand they search the original unions for their underlying faults. Guess what? within a year she was griping about his faults and inconsideration.
Maybe if she had seen his reaction and any display of virtue or lack thereof during an annulment proceeding, that problem would have solved itself and she could have called the wedding off. Instead, Catholic “Rules” became the enemy. And she ended up married, but miserable.
Always best to follow the Manufacturer’s directions…
