An Unheeded Warning

Cady's parents died before she graduated from college. They left her with enough money to get her graduate degree and to buy a nice four unit condo in the suburbs. To christen her new place and celebrate her new job, she throws an enormous bash. Unwisely the night before her new job. Prior to the big bash, Cady made a point of meeting all the other condo owners and was pleased with the two she had.
Nettie an older lady who lived alone and Patti, a young girl slightly older than Cady and mildly autistic, she thought. It wasn't long before she learned that Nettie was the busy body around the place and Patti was a bit of a strange lurking type that gave Cady the chills. The night of the party Cady and two of her female friends sneak off to her bedroom during the party and show off their lesbian skills and expertise.
All three girls are breathless by the time the party winds down, and as Cady turns out the lights she spies Patti peeking in the bedroom window as though she had had front row seat for the ad hoc threesome. This sets in motion a plan to teach Patti a lesson, a lesson that Cady felt would discourage the peeper from her perverted ways. But Cady underestimates Patti's resolve and realizes she may have her hands full with Patti.
And is at a loss of what to do about it.