A straightforward title indeed! That face she gave him on page 8...So alluring and yet somewhat...dangerous, almost. Getting some real yandere vibes from her, especially when she mentions him making her do something for the first time? I'm not sure if she's referring to some unknown backstory we have yet to see between them or she's kind of making it up on the spot...Or is she referring to him taking her in and into his room? "You're the one who took me in so own up to it and take care of me"? I guess if you actually called out to her and offered to let her stay the night, then it's your responsibility to take care of her for the night, yeah... That ending though. "I'm a part of your family now, I've decided to become your pet"? Erm...My first guess/suggestion would have been wife, but uh...I guess pet works too. Now I'm curious as to where this series is going to go. Will his sister find out? How would she react? Especially since they made it clear to us they aren't related by blood...
it's just guesswork at this point. just didn't feel like a yandere story at all. from the setup, the way she behaved herself - everything. i mean, they made the sister a "step" one for a reason. so there's yandere potential there if the author wants.