Grave Secret by Charlaine Harris

Harper Connelly is in north Texas working for wealthy Lizzie Joyce. Lizzie wants Harper to tell her what her deceased grandfather, Rich, died of. Harper's news that it was possibly murder and that his young caregiver, Mariah died in childbirth, confuses and angers the family. Harper and her stepbrother/boyfriend, Tolliver, are also in Texas to see their young half-sisters, who were taken away from the family when Harper and Tolliver were teens (because of their respective parents' drug use). Seeing their sisters always brings up mixed emotions for them, given all the bad memories dredged up, especially the disappearance of Harper's sister, Cameron, who's been missing for years. Tolliver's father, Matthew, has also recently gotten out of prison, and is trying to re-establish contact with him. When it appears that someone wants Harper dead, she wonders if it's because of the Joyce case or a more personal matter.... Grave Secret is the fourth book in the Harper Connelly series.