Nowhere within the CCC does it state that Patty would be committing a sin by merely attending this wedding, and the marriage will take place whether she is there or not. While it’s great to take a stand for your own principles, if this creates a rift(which it almost always does) then Patty is the one ending the relationship. Once the relationship is destroyed, her sister is much less likely to ever return to the Church.
We’ve got to take a step back and look at the big picture here, since this is just one small moment in the journey of our lives. Let’s say Patty swallows her pride and comes to the wedding, even if her sister comes back to the Church at the age of 90, it would be well worth it. If Patty doesn’t attend and causes a rift in the family, her sister will never return to the Church even if she wanted to, because that would be admitting that Patty was right(pride is an ugly thing).
Besides, if this is her second marriage, the odds are that it won’t last anyway(look at the statistics). If Patty is there to help her through the divorce, it would be a perfect opportunity to bring her back into the grace of God. Ask yourself, which decision are you more likely to regret? You lose nothing by attending(other than a little pride), but you could lose your sister forever by staying home and doing nothing.
As for me, I thank God every day that I attended my brother’s wedding, even though the easiest thing would have been to stay home. The hard thing is to put yourself through watching your loved one distance him/herself from God’s truth, but it’s well worth it if they will one day return to the Church. We are a people of hope, God would not abandon us if we sinned, so why should we give up on each other?
