First is an incest story where mom, son, and daughter hookup. It'd be awesome if they were overcoming something.
Second is a story where the girlfriend feels she needs a girlfriend. Maybe she feels guilty about it too. Not that she is doing it for her boyfriend. Bonus if it is a brother / sister couple.
For the first story, I enjoyed Ombligo's "Accidental Lovers"
The second one is almost-but-not-quite the case in my story, "Building a Better Past". Lazlong's "Help!" is similar as well. In neither case, though, does the girlfriend feel the need for a girlfriend, instead they find they are in love with a girl in addition to the boyfriend.