So logic... MC does what sister girl says cuz hes a typical selfless protagonist, checks out.
Sister is happy MC does what she wants no matter how bullshit her hypnosis is and it looks like she knows its bullshit since she's constantly half arsing it, checks out.
This detective girl wants MC because shes randomly decided she loves him more than life itself so she devices a big plot to get hypnotised by nonfunctional hypno and this will somehow put MC into moral debt to her and bind them together forever, no wait what? How does that work, where's the logic? I don't see it
Reading between the lines — my theory is MC-kun does it to protect his relationship with “imouto” because he loves her (he just hasn’t figured out what type of love it is because of their stale relationship so he’s lying to himself to a degree.)
“Imouto” knows what she wants but feels like she can’t ever show it because they are “siblings” so was hostile to him as a cover while secretly harbouring feelings (and self inserting into that bro-sis incest book series). Hypnosis lets her get what she wants without the fear of rejection and losing him for real
Osananajimi-detective girl has always loved him in a genuine way since they were kids and been forward about it but knows she is being stealth cock/ovary-blocked because of MC-kun’s ambiguous feelings for imouto. She is implying here that imouto’s hypnosis to make MC-kun love her didn’t work because he probably already loves her (which is extra painful for her given how long she has also loved him and always been upfront about it but never taken seriously while imouto kept everything in the shadows).
legit didn’t see him hypnotising her on the bingo card though, but I’m guessing that it won’t work on her for the same reason imouto’s didn’t work on MC-kun