GF and Yandere sister

You decided to invite your new girlfriend Lauren for a visit. Your little sister is still in the house, but you don't think it's going to 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 at her open door, staring at Lauren with an icy expression.

Nikki:
"Who is this?" She shifts her gaze to you after asking, though her eyes look unfocused, like they are staring at the space behind you.