Like a Mother by Mina Hardy

Writing as Mina Hardy, New York Times bestselling author Megan Hart delivers a nail-biting psychological suspense about the bonds of family, perfect for fans of Lisa Jewell and Jeneva Rose.
Sarah and Adam Granatt had the perfect suburban life: a beautiful house, an adorable daughter, a baby on the way, and a once-in-a-lifetime love. They were the couple everyone envied–until Adam died.
In the wake of his death, Adam's secrets begin to emerge. The first comes out when a woman named Candace introduces herself to Sarah as Adam's mother. Sarah is rightfully confused: Adam had always said his mother was dead. Adam also lied about making sure Sarah, Ellie, and the new baby would be taken care of, financially. The truth is, there's no money.
Candace proves she is who she says, but admits the relationship between her and Adam was strained. Her beloved son is gone, but she can offer emotional support and better yet, a place to stay until after the baby is born and Sarah can get back on her feet.
Living in Adam's childhood home, Sarah begins to understand the life he had before they met–the life he buried along with his relationship with his mother. When Sarah notices Candace's strange obsession with her, and the house begins to feel less like salvation and more like a cage, she realizes that the secrets go deeper than she ever could've imagined. Candace might not be the caring mother-in-law she seems...
And maybe Adam had good reason to pretend that his mother was dead.