My prediction is that Mashiro's target was always Yuhi's sister, but the sister might have been unobtainable for some reason back then, so she used Yuhi as a more easily controlled replacement. However, it's possible the sister later became obtainable, so Mashiro dropped Yuhi and kept their relationship a secret from the sister. Yuhi is of course too ashamed/afraid of being blamed for everything to tell anyone about what happened back then.
As for Aoi, my guess is that she's married or something. Being called "Miss NTR" point blank tells you she's not a good person. She doesn't seem to have any particular concern for Yuhi either. Unlike Sakurako who focused on helping Yuhi through her issues, Aoi just told her to forget things, so she could sleep with her. Then when Aoi was done with Yuhi for the night, she quickly bailed on her. She also wants to keep her personal life secret from Yuhi, so I think she's already in a real relationship with someone else and is just using Yuhi to blow off steam.
Any more panel time from Aoi is likely to serve the purpose of yet another blow to Yuhi's self-esteem, to show just how unhealthy her friends with benefits relationships are. Because Yuhi chases away anyone who genuinely cares, which leaves her with only women that use her as a convenience for them.