Sis is NOT dating MC
Vicky is NOT dating MC
Only Nicole might be, unless you broke up with her.
Nicole is the ONLY GF
Nicole is the only one "Dating" MC at any point throughout the VN up to current ending point.
So Sis and Vicky finally discuss things and discover they are both riding your dick, but it is not OFFICIAL or serious with EITHER of them. That was part of the point of the whole conversation was that maybe they do care enough about MC they want to be official. But Vicky and Sis also have a past and latent sexual attraction to each other so they decide to get some O face from each other. Neither one has said I am ending my dalliance with the MC over this. In fact, Vicky very CLEARLY states:
I promise that whatever happens once we go into the bedroom...
Won't change anything between us or your relationship with MC
That direct quote ELIMINATES any Netorare, or in English since this isn't fukkin Japan, losing a Love Interest through sex with another!!!!!!!