High Tide Club by Mary Kay Andrews

Elderly heiress Josephine, dying of lung cancer, calls young lawyer (and single mom) Brooke to her island estate. She wants Brooke to save her island from being turned into a park by the government--and she wants Brooke to find the heirs of her oldest friends, the High Tide Club. Strapped for cash and kind of intrigued by Josephine, who is a handful, she accepts the job. Once she starts researching, she realizes that of the original group, only Varina is still alive and that Brooke's own mother is the daughter of one of the members of the original group, Millie. That makes it a conflict of interest for her to work on Josephine's will or any of the bequests she has in mind. So she brings in her old mentor, Gabe.

Part of the story is told in flashbacks of the weekend when the High Tide Club was born and the aftermath of that eventful weekend, which involved the disappearance of Millie's abusive fiance, whose fate was never discovered. It's quite the page-turner, with a lot of colorful characters.