To be honest, when I see this kind of manga, be it a normal or a Yuri one, the sadistic part of my brain turns on and I humiliate the MC in my mind. And laugh with the imouto...
But thinking in a neutral perspective, this got me interested because first of all, I can't predict what might happen next. No doubt that Onee-san will be humiliated further and bound to the Imouto but after that, no idea.
Well, I'm quite interested in the actions of Imouto-san so I'll see if I shall continue this or not... Imouto must keep me interested or else... I will personally go there and attach a collar on her, the same as her sister's. So they'd be matching...