The sister will sexually assault the FMC in chapter 2 and take pictures, and will most likely use that to blackmail the FMC in the future so that she can keep having her way with her behind her brother's back, so this manga will basically be yuri blackmail ntr (unless the brother is in on it too, but that is still unknown)
EDIT: the author has stated multiple times in his Twitter/X account that this won't be a yuri manga. But in my opinion it will still have some yuri elements with the whole situation of the sister training the brother's gf to be better at sex to please her brother, so yeah if you can't stand any sort of yuri elements this manga is not for you and at the same time if you expect this to be a yuri story this manga isn't for you either, either way it's a trainwreck