It seems my theory that the sister training will make the girl dislike sex with the dude can actually be true.
Sister will train her to supposedly help her bro have a living sex doll, but it will backfire cause the bro is typical quick shot/stumbling in bed, which will end up with him not satisfying her, while the sister training will. Thus girl will probably get NTR by sister.
My only question is - is this actual goal of the sister, aka to steal the girl from her bro so that bro will be devastated or something and be "hers for ever" in some fashion - or will this be her mistake, unforeseen consequences of her training, and it will mind fuck her as much as the poor girl.
Good ending will be all 3 of them having 3some orgies. Cause for example sister got wind of girl being unsatisfied/her training failing its goal, and thus deciding to train dude too, and cause everyone had sex with everyone - it would end up with all 3 of them fucking.
Bad ending will be either all 3 of them broken and sex-less, or dude sex-less and sister being targeted by girl who gone full lesbo for her after all the training.