The Pink Motel by Carol Ryrie Brink (1959)
Nothing exciting ever happens to Kirby or Bitsy Mellen–that is, until their mother inherits a motel in Florida from her great-granduncle Hiram, complete with a roster of eccentric guests. New this winter are the Browns, obsessed with their suntans; Miss DeGree, obsessed with her poodles; and two mysterious men, obsessed, like Kirby, with trying to find Uncle Hiram’s rumored secret treasure.
For the 1959 original book, available used at amazon.com, click the link The Pink Motel
For the 1993 paperback version of the original, new or used from amazon.com, click Pink Motel