Witness at Large — Mignon G. Eberhart

Blood On The Beach
They had gathered on the private island to enjoy the summer and each other's company. But that was before "the accident," before the pretty young girl called Sister found Tom on the pier, crouched over Mildred's lifeless and battered body. Now as the handful of friends hide evidence to protect one another, the killer in their midst plans his next move. Is he Tom, the man Sister has secretly loved for so long? Or Boyd, who has been a brother to her? The list of suspects is short, and with the killer closing in the tiny island is getting smaller all the time.