This topic is, unfortunately, not new.
The short answer: so far as anyone has been able to find, Holy Mother Church has not definitively defined attendance at such a wedding to be sinful.
Here’s the bottom line so far as I’m concerned (and it happened in my family, so I know whereof I speak): Do you intend to have a relationship with them after the wedding?
Half the answers you will get would say that your presence at such a wedding is tantamount to supporting their rejection of the Church, so send your regrets and stay home. Explain your reasons if you think such an explanation will be received. If enough of the family objects, and they really care about family, they may decide to get married in the Church after all.
The other half of the answers you will get would say that harsh positions often backfire. Attend the wedding; your presence in the pews is a sign that you want to work on building bridges with the hopes of having them return to the church. After all, Jesus didn’t insist that sinners be made clean before he accepted their hospitality and ate with them.
Ultimately it’s your call. There are legitimate reasons for either answer. However, if you do attend, limit yourself to staying in the pews and not participating in the wedding party. Active participation in a non-Catholic religious ceremony would be indicative that you accept their “new” religious lifestyle.