The Glass Wives by Amy Sue Nathan. The book is being released by St. Martin's Griffon on May 14, 2013.
Amazon says this about the book: Evie and Nicole Glass share a last name. They also shared a husband.
When a tragic car accident ends the life of Richard Glass, it also upends the lives of Evie and Nicole, and their children. There’s no love lost between the widow and the ex. In fact, Evie sees a silver lining in all this heartache—the chance to rid herself of Nicole once and for all. But Evie wasn’t counting on her children’s bond with their baby half-brother, and she wasn’t counting on Nicole’s desperate need to hang on to the threads of family, no matter how frayed. Strapped for cash, Evie cautiously agrees to share living expenses—and her home—with Nicole and the baby. But when Evie suspects that Nicole is determined to rearrange more than her kitchen, Evie must decide who she can trust. More than that, she must ask: what makes a family?