I received invitation to attend my sister's wedding this December. However, this sister went off the derech many years ago, leaving Yiddishkeit and religion altogether behind. She is now an athiest, and will be marrying a non-Jewish athiest in a overall non-religious wedding ceremnony.
May one attend such a wedding? My gut instinct is no because it might be maaris ayin that you are encouraging intermarriage. Might it make a difference if the guest is closely related to the bride/groom?
This question discusses an engagement ceremony, but none that address my specific case with a wedding ceremony.