The Crypt, by Jonas Saul

I had a tough time figuring out what was going on in this novel.  It turns out it is number three of a series.  The hero is Sarah, who at times will go into a trance and write messages from her dead, but apparently well informed, sister.  This makes her a target of a US government agency who wants to examine her abilities.  She prefers to not play with them.  Lots of other folks want to kill her.  The ending was both unclear and unsatisfying, but maybe that's because there are more books in the series.

The Crypt (The Sarah Roberts Series Book Three)