No! Why would you even want to attend? This isn't a matter of showing basic respect, ie. attending a non-Catholic funeral for family or professional reasons. It's a matter of celebrating mortal sin. You probably can't do or say anything to stop the people; they're going to do what they want. My best advice to those who know that voicing or otherwise making the truth known will make matters worse, drive the people further into sin, is to ignore the so-called marriage and avoid those involved as much as possible. Treat them as you would strangers. If you must interact with them, be cordial, be polite, be brief, and be gone!
If this means cutting family ties, so be it. Replace them with friends of sound moral character. If you can't find any, keep to yourself and give more time to God.