The sister being interested in here is weird as fuck. The MC is already being cucked by others, why is she accepting a request to wear an outfit for a random guy she just met? I get it's for her "research" but it's still a weird thing to comply with so easily and the MC just allows it?
She acts like a toddler who doesn't know how to socialize and doesn't understand her own feelings.
The first couple chapters were fine, felt like it would be a good story. But these tropes are spelling bad news for the future of the manga.