The Replacement Wife by Darby Kane

 Elisa has always gotten along with her brother-in-law, Josh. But lately she's beginning to wonder if she knows him at all. First his wife Candace dies under mysterious circumstances, which the police rule an accident. Then Elisa's friend Abby becomes engaged to Josh--then disappears without a trace. Then Elisa finds out about another woman in Josh's life who met an untimely end and wonders why no one except her thinks this is suspicious. At the same time, she's dealing with a trauma in her own life that has left her stressed and depressed. When Josh shows up with a new girlfriend named Rachel, Elisa begins to panic that something will happen to her too. But Josh keeps trying to convince everyone, including Elisa's husband, that she is getting paranoid and suffering from mental health issues. 

You can feel Elisa's stress level rising as she tries to investigate what really happened to the women in Josh's life--and protect Rachel, a woman she's not really sure she trusts.

I could see why her husband took the brother's side--especially because he had raised him after their parents died. But even he had to see that an awful lot of women who dated/married his brother had come to unfortunate ends and maybe his wife had a valid reason for being concerned, especially since Josh loved to spend time with their small son.

A quick read and a good psychological thriller.