The Murderer and the Lost Treasure by Leonardus G. Rougoor

Siblings Matthew and Elizabeth Janssen, nineteen and eighteen, live with their parents in Lilac Cottage in Cape Cod where they had been vacationing when they discovered a time-travel grandfather clock hidden in a secret room. When their parents bought the cottage and everything in it, including the clock, Matthew and Elizabeth are elated at still having access to the time travel device. Summer vacation is over, and the two teenagers now have to plan their adventures around school and family life, making it difficult to get back into the past to solve mysteries. And, boy, do they have mysteries to solve! Not only have a number of young girls been kidnapped and murdered between 1914 and 1927—crimes the siblings are sure were committed by the same man—but the jewels from a jewelry heist in 1900 are still missing. When the siblings find a riddle written by the man who allegedly masterminded the jewel heist, they are sure that if they can just figure out the clues, they can recover the missing treasure…that is, if they can survive their dealings with the murderer of the young girls.