Thank you so much for the scanlation of the main oneshot and the prequel. I wasn't aware there was more to the story.
I am not entirely sure what the final goal for Sensei is. She loved her sister, and now her student somewhat reminds Sensei of her sister. There is something really twisted going on here. She wants revenge from her sister's bully by defiling the bully's daughter, yet it seems that Sensei has fallen in love with her student too (In a yandere sort of way).
I guess the best revenge that sensei could have is to steal the bully's daughter, lover her and have her forget about her mother. We need an epilogue.