Normally I'd be upset with the MC for rejecting (former) best girl, but since we know now that they're not actual siblings I don't care. Back to square one I guess, best girl is the one with biggest boobs aka masochist senpai.
I have bad news for you, my man: from what we've seen so far, deep down you're just as much of a pervert as these girls here. So get off that high horse of yours, it'll do you good.