Since most demons are women, I suppose it will be the sister who dies (harem bs). The question is why make a contract with a demon that forced his sister to die? Knowing that her soul has ceased to exist? To get revenge on whoever killed the family in the first place? Nah, first on the revenge list has to be Venus. If he somehow ends up liking Venus, this will definitely end up on my dropped list.
Thanks for the chapter!