You might like to get your trademark lawyer to send them a brief note that in the event of any legal action being taken against you, you'll be forced to pursue trademark cancellation, and in his opinion they'll lose their trademark entirely if this happens, so it's probably better for everyone if they just leave you alone.
The issue was broached in our response, which follows:
As you well know, the text you quoted refers to a TV show on the TLC network. It has nothing to do with your client and anyone that bothered to read the sentence, even a few words prior, would know that.
Every single reference to the term "sister wife" or "sister wives" (which is multiple wives in a polygamous relationship) is in reference to people that are commonly called that or specifically named TV shows. It is not ever, not even once, in reference to your client's site, which was named to take advantage of the recognition of a commonly used term.
This is clearly demonstrated by the TLC show Sister Wives, which premiered in 2010 and by the Israeli documentary Sister Wife which was released in the year 2000 as well as many other uses in media and historical use of the term in the polygamous community.
We want nothing to do with your client. They have a horrible reputation in the community and we have nothing to gain by being associated with them in any way. The last thing we want is to be confused with them, as I think is clearly demonstrated in our business practices.
We have, on two separate occasions, consulted with a trademark attorney in regard to your client's frivolous complaints. Both times, we were told that we were in no way infringing and that your client should never have been issued a trademark. We were advised that we could start a cancellation proceeding if we wanted to pursue it. We chose not to, though that may be something we need to reconsider.
However, If you find anything that actually could be construed as confusing by a logical and coherent individual, we would absolutely be glad to remove or correct it as we do not desire any confusion between our brands any more than your client does. Use in a legally allowed descriptive or definitional sense will not be removed.
Any further attempt, as your client has already done, to disrupt or damage our business with unfounded claims to 3rd parties will result in legal action against your client for damages.
If I could be of any further help in this matter, please let me know.
Thank you,