Moon Over Edisto / Beth Webb Hart.

Enhanced descriptions from Syndetics:

The past has come knocking on Julia's door. Can she summon the courage to answer betrayal with love

Once, they were the happiest family under the sun, crabbing and fishing and painting on beautiful Edisto Island in South Carolina's lowcountry. Then everything went wrong, and twenty years later the Bennett family is still in pieces. Mary Ellen still struggles to understand why her picture-perfect marriage came apart. Daughter Meg keeps a death grip on her own family, controlling her relationships at a distance. And Julia thought she left it all behind.

Julia's best friend, Marney, broke up her parents' marriage years ago. Now Marney shows up at her Manhattan apartment, asking the impossible--come home to Edisto Island to care for the half-sisters and half-brother she has never known. Marney, recently widowed, has lung cancer. There's no other family to care for the children while she's in the hospital following surgery.

Julia loathes Marney. But if she doesn't step in, her own mother--who has never gotten over the divorce--will be called upon to take care of the children. So Julia heads to South Carolina to keep the peace.

Julia grudgingly agrees to stay a week caring for her three young half-siblings. But there's something about Edisto that changes one, and she begins to reconnect with the place and the people that she's been running from her whole adult life.

Can Julia and her fractured family somehow manage to come together again under that low-hanging Edisto moon