@muge - Yes, an infographic about these characters might be certainly helpful. Thanks and sorry for all the trouble.
I certainly feel like Shimura could have done way better for these characters (Mayu's cousin and her bf) way back when and not 4 or 3 chapters before the series is ending... the plot of this chapter feels like it was somewhat cramped and rushed. And too simplified for such a thing like this :/c. Then again, iirc, Mayu's cousin was very impulsive so, I guess... yeah... At least she didn't end up with her mom's bf and the guy was honest with her about that, too.