This is a situation that more and more Catholics will be facing as their family members go along with what the culture has deemed okay.
You should start by communicating that, despite what the culture says, your faith says that marriage is between a man and a woman, so you cannot approve of this union. You could maybe find some way to connect with them either after the ceremony to express your love. (An appearance at the reception may be possible but could send confusing signals to others.)
You would have to clarify why you were late and missed the service — and then perhaps get into unpleasantries with others explaining why you are not in support of gay marriage.
Perhaps you can make a date to get together with her or them later. Jesus never said this was going to be easy. He said that sometimes a sword may divide family bonds. (Matthew 10:34-36)
You should be prepared to defend your faith as to why marriage is for the complementary sexes, and why attending their ceremony is being a witness to their union, and therefore not possible for you.
Your sister may not understand your adherence to this code of faith, but if you do it lovingly it could witness to her.
I wish you the best.