go to the wedding, with a date (preferably an older, prettier woman). be all smiles and laughs. introduce the date by name only. avoid the topic of whether or not your in a relationship with her. don't talk directly to the ex (but don't be rude, if she bombards you with her presence, just say "hi" and move on). skip the reception, but leave an envelope with a couple c-notes, and a card (for the couple, of course), with the mom before you leave the church. duck out quietly but don't sneak out. you've shown the ex's family you care about them and like them. you have not shown any disrespect to anybody, yet most likely irked the s*** out of the ex and have her second guessing her choices. go to nearby bar. get date blind-drunk and enjoy generously. repeat.
