SOLVED. The Saturdays by Elizabeth Enright about a girl named Mona (blond hair), her 2 brothers and sister. Move to a Victorian style house with a cupola. Find a hidden room above piano.

[Vee]

YA Mystery Book about a girl named Mona, her 2 brothers, and a sister. The youngest brother gets hurt and almost crashes into traffic when riding a bicycle downhill. Officer (Murphy?) takes him to his wife's house. They moved into a victorian style house with a cupola. They have a piano in their attic and they find a room above the piano. They might have found a part of an old spinning wheel in the room. A message leads them to search for more clues. They find a message in a flower pot by a mailbox. The fat housekeeper has to sit on a suitcase to get it shut. The youngest brother may have been named Randy. Read around 2017

[Kris]

Vee, I notice your request is in a Solved folder. Did you find your book - or should we move your thread to the "Unsolved" folder?

[Orsolya Könyvtáros (formerly Memp)]

This one is actually The Four-Story Mistake, by Elizabeth Enright. The Saturdays is the first one in the series.

[Kris]

Ah, I just noticed the title in the header. Thanks Memp.