Only you know the full situation, of course but based on what you say, I urge you not to stay away from your daughter's wedding, even if you rightly disagree with the way she's been living or feel she is making a mistake.
After all, you love your daughter and want what's best for her; if you didn't, you wouldn't be concerned about her impending marriage. But if you refuse to go to the wedding, almost certainly you will be cutting yourself off from her in the future. Do you honestly want to do that? Our children don't always make right decisions but they need us just the same.
In other words, right now your daughter is determined to go her own way but what of the future? If life turns against her, will she feel free to turn to you for the help she'll need? Don't close the door to the future. The Bible says, "We who are strong ought to bear with the failings of the weak and not to please ourselves" (Romans 15:1). The Bible also tells us to "help the weak (and) be patient with everyone" (1 Thessalonians 5:14).
Most of all, pray for your daughter and her future husband, that they will realize their need of Christ. Only He can give them the stability and strength they need for their life together, and only He can change their hearts and attitudes.