YA Mystery novel, short. Two rich teens and their inheritance. Early on, scary looking man kills another man in a museum.

[Drew Sims]

Read it as a kid loved it but all I can remember is that it begins or very at least early in the book a scary looking man killing another man in a museum with a knife describe as so thin that it did not leave a trace on the skin. There was also two main characters a boy and a girl possibly related possibly not. Possible series although I only remember one book. I also remember something about a gem or treasure of some kind.

[Kris]

Drew, around what year did you read this book? A range of years is fine.

What is the story's time period and location?