The AAA link it great, and as it says at the bottom, going to a same sex ceremony is not to be recommended. It is very different from even attending a problematic heterosexual marriage.
Now on to my advice - not that you have to do this, but we are called as Christians to witness to the truth. While you COULD decline to attend without telling the real reason, you could be
a-missing an opportunity to witness to the truth
b-giving them a chance to misinterpret/wrongly judge the Church because all they have to go on are their assumptions.
I recommend writing out a letter to your neice. Even if you want to talk to her in person, having something written will allow you to be sure you didn’t miss anything you needed to say, and also give you something to give her to understand. We had to write a letter like this once (not for a gay ceremony, but an invalid hetero marriage) and this is how it went.
Dear ___:
-Small talk
-We love you, we want you to be happy, but most of all, we want what is best for you.
-As you know, we are very committed to our faith, and try to do our best to live our lives according to our Church’s teachings…
-The Catholic Church teaches ___ about homosexual activity and ___ about same sex marriage.
-Because of this, we can not attend your ceremony. I hope you can understand that it is impossible for us to go against our Church’s teachings on this when following Church teaching is so important to us.
-We are not judging you. We do wish that you would reconsider your decision. We do love you, but as I’m sure you realize, we also have to follow our consciences.
-God Bless you. Love, ___