I edited the overview, let me know if its still too spoiler-y.

===SPOILERS AHEAD===

The bit that I feel contains spoilers is the vampire subplot involving Emily, which isn't even alluded to until halfway through the film. I'd do something like this:

Newly released from a mental ward, Jessica moves to a country house with her husband and close friend, only to find a mysterious transient girl living there. After a series of potentially supernatural occurrences, Jessica's grip on reality begins to slip again, escalating when she learns the disturbing local legend of the home's previous owners.

I think this is a decent synopsis of the first act or so without going into too much detail.

Wikipedia has a thorough plot rundown, for reference:

https://en.wikipedia.org/wiki/Let%27s_Scare_Jessica_to_Death#Plot

I removed the bit about the vampire.