It was an adventure/action/mystery book that was well at least 300 pages long. It followed these two brooding-looking dark haired twins (one girl, one boy) who were exploring the mystery of their family history? I think it had something to do with their father specifically? There was a moment with a cliff I think, and a tiger! But it was very wacky and so much was going on. I read it in elementary school and it was my favorite book ever.
Lost Classics of Teen Lit: Name That Book! 