Réponse :
The note said : "Mark Crimson. He broke your window."
Explications :
Why ? Well, it's quite surprising at first sight. But, with Sherlock Holmes, everything can be tricky. However, that riddle is well-known. If you look closely to the note, you can read that the one who broke the window is ?. What is important about that simple question mark. Well, I guess you now understand the little note.
? is a question mark.
The three brothers are named John Crimson, Mark Crimson and
Paul Crimson
Well, I hope that makes sense, now. Good luck, young Sherlock Holmes !