Should this even be considered incest? They call themselves "sisters" but they're not actually related at all.
Well if characters like Lilith Aensland and Morrigan Aensland are called "sisters" despite Lilith literally being Morrigan, well a portion of her, then why can't these two be considered siblings?
No...
It's just because you're fingering me in such a naughty way... Ah!You're pretty weak right here, aren't you sis?Okay, okay.When this is done, I want you to do me too, okay?I'll do it, so keep going... harder, right there...!