@spandxltd The point of entrapment as a legal defense is to suggest that the crime was only committed at all because the person was somehow coerced into committing a crime they otherwise wouldn't have done on their own. Demanding that some guy sells you his prescription pain meds despite his repeated refusals until he finally caves and does so is entrapment. Being undercover and getting propositioned by someone while you're minding your own business just because they think you're not a cop or such is not.
If we really want to pick at that picture, it's better to point out that no crime has actually even taken place at all yet. She could be talking about anything. We're assuming it's sexual because of the premise of the joke.
@Glomoro That depends so much on the country, my man. Besides, what you describe is closer to coercion, than entrapment in most places. For example, back here the police can't actively try to get you to break the law, even if threats or demands aren't used (because honestly, it's pretty fucked up morally).
As per the contract, Chiyo will be Yuu-kun's sister in exchange of something he values most.
...Considering that he yearns for family (and therefore Chiyo, eventually) more than anything, his payment might as well be in his newfound sister suddenly being gone after living most of his life with her.