I am looking for a novel about a married woman who has come into contact with her ex boyfriend. One day at the beach with her child, stepchild and pregnant sister, she is communicating with the ex when her children get dragged out to the ocean. She runs in after them but can only grab one and she chooses her own child. Her sister, a strong swimmer, saves the stepchild but needs to be hospitalized due to bleeding.

There is a beautiful sentence about the bond between sisters.