I mean, you are also stranded with them there, with no way out currently, plus there's five of them and one of you, at some point you would drop your guard and they could surprise you, if they get too suspicious of you not playing ball. MC was basically coerced into getting involved with all of that during the game, not overtly, but i think MC realises that if kept the attitude of yeah i'm going to report you all to cops once we get back to civilization, they might just get rid off MC instead. Considering what they've already done to preserve their secrets, i doubt being sister or lover to MC would be enough to prevent them from taking action. Only other option for MC to stay safe i could see, but without getting any information, in there would be trying to leave and survive in the wilderness, but i doubt MC is really seasoned outdoorsman.