Lily,
You sure have done your part to make your community a more social place, but these people are not worth your efforts. Their friendship is so shallow and they are so self-righteous that they are miserable in their tiny lives that they resort to belittling others to elevate themselves. Grow up, people!
You might be well served in finding another group to socialize with. Do you attend a church? They often have social functions, and there is opportunity to find like-minded individuals there. Do you play cards? Euchre and bridge clubs are another source of networking. Do you knit? I could go on, but you get the idea. You need to keep shopping for the group you fit into.
Older women feel very threatened by younger, single women. They think every single woman is after their man! Let them have their insecurities, and YOU go out and have a good time with YOUR man!
You have every right to celebrate this relationship and not feel that it is tawdry. You are not seeing a married man, right? But do keep your eyes open, and make sure that the sister doesn't continue to control her brother's life. He needs to have enough of a backbone to tell her to back off. If he doesn't, more problems will lie ahead.
For now, in the words of Bonnie Raitt, "Lets give them somethin' to talk about!"
Karen