Look, MC; she isn't blood related to you.
The taboo to prevent birth defects does not apply.
And she desperately wants you to plow her like it's Nebraska in April.
You literally just banged the living shit out of hundreds of women, and it's plain you've got little-to-no emotional attachment to her as a close family member (or you wouldn't have taken a year off to plow most of Amazonia).
So, seriously; just stosh your not-really-a-sister, already.
And if you won't, then at least send her over here, cos dayum...
(Yes, I know he'll never actually do it, because after hundreds of women, he somehow has the "never had sex" mentality)