I think there were actually three scenes with Aiko. Note that it was established that Aiko's mom had some special relationship with Mrs. Lee, and thus wasn't at risk of suffering the fates planned for Shizuka and Asami; thus, no need to rescue her.
In story terms, Aiko was a opportunity for the MC to have sex while the friends and family members warmed to the idea of incestuous relations and a polyamorous boyfriend.
In story terms, Aiko was a opportunity for the MC to have sex while the friends and family members warmed to the idea of incestuous relations and a polyamorous boyfriend.