Of course not... She makes Virginia answer it.

"Oh shit, Virginia, can you answer the door? I need to go get something real quick!" And before Virginia could respond, Sara ran off up the stairs.

Now, it was quite possible that Virginia wouldn't answer the door, or even for her to chase Sara down instead, but Sara bet on one thing, since these were Virginia's friends, she just might panic and let them in to not arouse suspicion.

once Sara arrived upstairs, she hid close by and listened in wait to hear what Virginia would do.

And in a few seconds, Virginia...