Oh boy. This is silly, but also has very dark implications.
I'm gonna give Shiori the benefit of the doubt and assume Onee-san didn't have his parents killed or conspire to send them way.
If that's the case, then it's mostly Minoryu's parents being awful people, to the point that I'm getting Hayate flashbacks.
Regardless, at this point I'm pretty sure he's better off living with her than with his own parents.