Oh man. I didn't think about it before, but yeah. Onee-san is gonna be super vulnerable to these sorts of scams with the big gaping weakness that is Minoryu.
Rigging things isn't the same as winning. It might share the same appearance and even the rewards of a victory, but if you're aware of things having been rigged you're also aware you didn't do anything with your own ability to get there. Sure, some people only care about the end result. Minoru is apparently not one of them.