In any case, the
older sister and the co-worker will still be something, won't they? Even if not ntr by "definition" will this continue?
Well, my guess the coworker (predatory bitch) took her first sexual v-card when sis first started working at the hospital...Maybe before she met and started dating Dr BF. And has some kind of dom control over her, being her first lover (MC said she was always incent growing up).
But I'm hoping if MC rejected that bitch's offer, he would have an opportunity to blackmailed her to say away from his sis. Or gets that bitch fired, or catches her fucking sis's BF to break her control over her.