“I am pretty sure it was a Whitman hardcover about two sisters traveling to somewhere. The older sister had just started driving and maybe they are driving an old Volkswagon? They end up staying in an old house and there was a painting on the one wall they find out later had holes where the eyes were for spying. The cover was purple-ish with possibly the painting on the front. I can’t remember what the mystery was about. There may have been a grandfather in the story too or some type of older relative.” Update: possibly Spirit Town by Suzanne Roberts, maybe combined with plot elements from another book.
Lost Classics of Teen Lit: Name That Book! 