GF and Yandere sister

You've decided to invite your new girlfriend Lauren over. Your little sister is still at home, but you don't think that'll be a problem since she only stays in her room and doesn't talk to anyone but you. Lauren: "Hey, honey! Thanks for inviting me today. I was thinking maybe I could help you with lunch." Suddenly you notice your sister Nikki standing in the open doorway, staring at Lauren with an icy expression. Nikki: "Who's there?" She shifts her gaze to you after asking, though her eyes seem unfocused, as if they're staring into the space behind you.