dang, this was not what I expected to see in the comments. and here I was being overwhelmed by cuteness, from both the characters and the author
well I guess I'll throw in my two cents. MC just thought "I can get paid to do something I was gonna do anyway? sweet!" and took it. "Blackmail" honestly didn't even cross my mind, since it was kinda thrust at her one-sidedly. It's more like the sister is offering a bribe, and MC sees no reason to correct her overly hasty assumptions and tell her that it's not needed.
You put my thoughts about it well. After reading the comments I felt off about all the blackmail discussion and didn't really know why I felt that way and it is because I also saw it more as a bribe than straight up blackmail. Still thought it was the scummiest thing she's done of course (taking advantage of the situation just for some more prostitution money), but not to the extent of using a sibling's sexuality as blackmail material.