Most of my husband’s paternal cousins are not frum. When we go to such a wedding we first check the caterer to make sure it is glatt. If it isn’t we get special meals. WE don’t go into the chupah if it is a reform or conservative synagogue that is being used, they know this. Our entire family, who attend the wedding, will go in to the smorg, to mingle with the family and give Mazel Tov, and then NONE of us will go into the ceremony, but will rejoin them at the dinner. My shver a”h who was their uncle didn’t allow it and we all followed the rules. If it is just in a catering hall where a chupah is set up then we participate. If it is a glatt caterer then one of us speaks to the mashgiach.
B”H we did not have an incident that any one of them married a goy. Although they are not frum, not even one considered marrying out of the faith. Our family history is very strong and public. Although the orthodox element is not necessarily there, the jewish identification, ideology and spiritual connection is very strong.
The concept of going to the wedding anyway if one marries a goy is probably to show the relative that they are a JEW and they remain a JEW and when the marriage fails, they will still be a JEW. Probably because we don’t accept the marriage and we expect it to fail.