Can Catholics attend the secular wedding of two people baptized as Catholics? I was born and raised Catholic, but became Wiccan a few years ago.  My fiance is Catholic, but neither of us wants a Catholic wedding.  We've decided on a secular ceremony, neither Christian nor pagan.

Most of the people in my mom's family are Catholic extremists.  They claim that because my fiance and I are both baptized, if we don't have a Catholic wedding then by Church doctrine they're not allowed to attend.  (As you can imagine, this is creating a lot of heartache.)  Is there any truth to this?  If not, is there any evidence I can show them that might convince them otherwise?
For the Church to recognize a marriage between two baptized Catholics as valid, they must follow canonical form:


  Can.  1108 §1. Only those marriages are valid which are contracted before the local ordinary, pastor, or a priest or deacon delegated by either of them, who assist, and before two witnesses…


Thus, you mother understandably does not want to commit a sin of scandal by giving any support to an invalid or doubtfully valid marriage.

The fact you and your fiance positively choose not to have a Catholic wedding shows ill-will on your part toward the only true religion.