I really don't see that happening in this game.
Jenna is quite possessive of the MC, she doesn't even want to share with her sister, so a stranger is definitely out of the running.
The part about Jenna not wanting to share with her sister, we don't know if that is actually true. Even CheekyGimp said that if you tell Katie the truth Katie breaks up with you but Jenna does not. So if Katie had not broke up with the MC Jenna would be sharing you with Katie then.
At first I was thinking it would not be possible to be with both Katie and Jenna either but the more I think about what Katie has said and how good and close the relationship was between Katie and the MC. That even if Katie breaks up with the MC they will get back together even if the MC is together with Jenna still.
If you really look at Katie I think she is as in love with the MC as much as Jenna is except Katie is more volatile which is why she got angry then broke up with the MC. But don't forget that she also got mad at her friends when they were coming on to the MC and got into a fight over it with Debbie but then she cooled off and said they are blood sisters and they share everything. So why would that really be any different with Jenna who is actually her blood sister.
Also in a way I see Katie being in the same boat as Jenna in that the MC helped her to change in a good way. No one has loved Katie the way the MC does. And of all the characters in the game I really believe that the MC genuinely loves both Katie and Jenna and the other girls are more of a fling.
I could see Katie's initial anger subsiding and then working it out with Jenna for both of them to be with the MC just like she did with Debbie.
Of course this is just my opinion and I am not the Dev so really just take this with a grain of salt. ( but of course Im still hoping that happens
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