Um... I doubt that will ever happen. If DreamNow (the developer) refused to keep Natalie and Roxy as LIs in the same playthrough (you have to have the MC choose one or the other), I strongly doubt he's going suddenly to add Rachel's sister as another LI, especially since even meeting her in the first place is dependent on having high love with Rachel. I guess he could let the MC abandon Rachel for the sister, but the two sisters genuinely seem to love each other. I think it would be incredibly out of character for either of them to backstab the other that way (or even accept the MC breaking up with one and then starting a relationship with the other). And again, DreamNow won't do incest (so no threesomes with the sisters or even both having relationships with the MC). So sorry, but I really wouldn't hold my breath for that, if I were you, unless you only want to look at pretty pictures of her not getting sexual with the MC.