Fuck her already. None of these incest manga have anyone remotely blood related doing anything, it's always "adopted". Just... get in there. Your girlfriend's a freak and your sister wants the D. Man. Up. Threesome, now.
@degenerateafro I suppose we could argue that the boss lady's particular actions constitute a sort of kiss, too. I mean, if a kiss on the cheek can be a kiss I can't help but compare the oral thing with a french kiss -1 mouth.