There's no explicit scriptural reason not to attend a wedding in a church, so why are they bringing this up? They claim scriptural principles apply, but provide none. If you go to the referenced watchtower article it links to a QFR from way back and the only apparent scriptural principles are:
- "do not become unevenly yolked with unbelievers" - How does this apply? If they're saying that you shouldn't go to a wedding in a church because there will be unbelievers there, wouldn't that also apply to any wedding of an unbelieving family member, even if it was held at the courthouse or in someone's home? This "principle" does nothing to make their point that you shouldn't go to a wedding in a church.
- "Get out of her my people" - I think we can all agree that this was not intended to literally mean to get out of a physical church building. Paul went to pagan temples to preach, so this scripture would condemn him as well if that were the case. There is no prohibition here on respectfully observing a wedding in a church. You're not joining up or donating money to false religion or providing any support or adherence simply by walking in the doors of the church building.
I have a scriptural principle for them. Do not go beyond the things written.
Edit: I guess I went off on a tangent a bit there, but this sort of thing shows them trying to go beyond even shunning DFed family to include breaking ties with family members that never converted. They influence the R/F JW to basically be a dick to anyone who's not a JW in hopes that any connections to normal people will at least die of attrition. So even if there's not explicit command to shun people that were never a JW (though lumping these things together will almost certainly result in some JWs interpreting it that way, and they know it) they are happy to try to get JWs to behave in a way that will result in their family cutting ties with them or otherwise "persecuting" them in order to keep the JW in the cult and "prove" that non-JWs are evil, selfish, and a bad influence.