Problem with her is that there is absolutely no reason for anyone to actually interact with her. Even more so after proving how mentally unstable she is to their faces while trying to manipulate the sister to her liking (and almost succeeding).
It's like universe bending over backwards to force her into the plot. Like she has some serious dirt on everybody and they simply can't ignore her even though ignoring her sorry ass is the best course of action. Just like how they walked out of the bathroom and left her there alone. That should be the ONLY move they should do whenever she appears with her room temperature iq schemes. Just walk away. She can do literally nothing and she is owed nothing.
Also, has the friend always been this obsessed with her (acting like a creep in the process desperate to get into the house) or are we simply inventing new reasons to include her to the story again?