HELP!!!

I am trying to find a book for a friend - she is sure she read it when she was a child.  Here is how she describes it:
The story was about a girl who has an invalid grandmother (or great grandmother) living with her family. The girl doesn't like her grandma because she is old and cranky and does nothing but scold. The little girl ends up finding her grandma's fort (I think it was a soddy underground.) and reads her diary from when the grandma was a child and the girl realizes that they were very similar.
Does that sound at all familiar?

Anyone, Anyone!?!?!

Thanks for the help - I am really struggling with this one!