"Your girlfriend had to leave quickly for work, leaving you alone with her younger sister Millie"

Sorry user, that's awkward. I'm sorry my sister just ran out like that, but she said I should take care of you until I come back.

"Slightly nervous about being alone with her sister's boyfriend for the first time."

"Millie sits down on the couch next to you." Do you want to watch a movie or something? It will be hours before she gets back.