For Karen to accept a harem route, Nikki will have to persuade her. That is the threesome that needs to happen.
Karen is supposed to be clever - running a successful company - she observed the change in Nikki when Nikki came back to live with her after the first summer when the MC was there alone.
How long before she realises her sister loves him too.
That will be the dilemma which makes her share.
Haley is not family - she will not want to break Nikki's heart, so she will agree to share.
Then it is a short step for Nikki to persuade her to be even more 'accomodating' with Haley, her shop owner friend, and maybe the doctor, and then the maid will also be brought in to the mix.
They'll need a bigger bed.
So I can dream can't I.