The Dandelion by Michelle Leighton

At thirty-five, Abigail Simmons has mastered only one thing—running. When tragedy strikes, she runs. This time her urge to flee brings Abi back to her hometown, where she hopes she can find the peace she’s always searched for. That hope is dashed when Abi’s ex, Sam Forrester, bumps his way back into her life.
Sam has flourished in ways Abi hasn’t. From the outside, he has it all—a successful medical practice, a beautiful daughter, and a charming wife.
Who is dying.
Sara Forrester is dying and her final wish is for Sam to find love again so she can meet the woman who will finish raising her daughter and grow old with her husband. Abi seems like the perfect fit, but what Sara doesn’t know is that Abi has a secret, a secret no one could guess.
And it’s a deal breaker.