Since I Don't Have You by Louise Candlish

They were friends who shared everything—until tragedy singled one out. At the birth of their daughters, best friends Rachel, Mariel, and Jenny make a promise: to love and care for each other's girls exactly as they would their own. Six years later and a tragedy has torn them apart. Within weeks, Rachel has packed up and gone. Settling on the beautiful, windswept Greek island of Santorini, she knows she has come to the right place, but as she slowly rebuilds her world she can't forget the pledge she once made to her friends. She hires a private investigator, the enigmatic Johnny Palmer, and arranges for him to send regular updates on the girls she has left behind. Over the years, with Palmer's help, she is able to secretly soothe their growing pains, as well as those of their parents. But in Rachel's new island life far from home, who will be there to guide her?