The Double-Dip Disguise — Cynthia Blair

Identical twins Christine and Susan Pratt spend the summer on idyllic Seagull Island with their grandparents. Chris works at an ice-cream parlor, while Susan babysits for the island's wealthiest family, who live in a spooky Victorian mansion. But both the house and the island are brimming with secrets, and despite the risks, the curious twins can't resist using their identical appearance to uncover the truth. Young Adult Fiction by Cynthia Blair; originally published by Fawcett Juniper