I had a similar concept with "Mom's Oral Servitude." In that story, the son sees his mom eating the daughter's pussy. The mom is mortified that they were caught, but explains that it happened by accident. The mom tried comforting the daughter one night, and she did the only thing she knew how to do in order to make the daughter feel better.
For a story like this, how about the daughter is eager to have the guy join, but the mom is apprehensive because she doesn't want the whole thing to get out of hand. The daughter discovered mom's secret fetish and demanded to join.
The daughter in this story is one of those outgoing girls who talks to much. The mom is prim & proper. The son is your average guy.
Mom agrees to let the son join, and the sister directs the whole thing. Perhaps the mom is tied up while the son spanks her or whatever. Then it goes to oral sex.