SOLVED. YA Mystery - Girl staying with Uncle for the summer tries to solve the death of the young boy who haunts the house they are staying in [s]

[Kitty]

While her parents are travelling for the summer, a girl and her younger sister are sent to stay with relatives for the summer. The girl is sent to stay at a lakehouse (maybe?) with her uncle (possibly older brother) while her younger sister goes to stay with an aunt elsewhere. I want to say that her uncle is a writer and is renting the house just for the summer? Anyways, the girl becomes convinced the house is haunted, and I believe finds out that a young boy was murdered/died in the house years ago. She chooses to stay in the boy's old room - i remember something about a green carpet and yellow curtains in his room. She can also feel the presence of whoever murdered the boy. *SPOILER WARNING*
(view spoiler)[ I remember something about stamps - like the young boy's father had a rare stamp hidden in the pages of a book that the killer was searching for. There was also a hidden room in the house? (hide spoiler)]

I must have read it in the early to mid 90s - because of the story I belive it was a YA book rather than a children's book.

[Ann aka Iftcan]

Kitty, do you remember anything about the killer? I am trying to remember a book similar to this where the boy's ghost is there, as is the killer's? And the killer's ghost is trying to frighten the girl away? Sorry, it's kind of vague.

[caracal-eyes]

Christina's Ghost - Pretty sure this is it.

[Kitty]

That's it! Thanks