Is there a person on the board of the masjid who you can contact? Maybe one of their wives? If there are halaqas that your husband goes to, perhaps you can contact someone who runs or attends them?
The only other thing I can think of is to have your sister email the imam too and say that it’s not only her husband, but it has come to her attention that there are brothers, maybe even sisters, in the community who are discussing their sex lives. If that imam gets enough emails, he could see it as a concern that needs to be discussed.