book about orphan siblings, mysterious person calling themself the Mason

[Lizzi]

Every time I tell my brothers about this book, they make fun of me because the details I remember are absurd. From what I recall of the plot, the main characters were two siblings (maybe twins?), one boy and one girl. They were mischievous, would do things like mess with traffic signs and generally be a pain. Then one day (this is the absurd part that I might be misremembering) walls start appearing overnight around town, with messages from a mysterious person calling themself the Mason.
I believe I listened to this as an audiobook, so the cover could have been changed, but I remember it was mostly black and white and the kids were wearing striped sweaters/shirts/outfits.

[Rainbowheart]

How old are the siblings?

Can you recall if the Mason was male or female?

Do you think this book was for kids, teens, or adults?

[Rainbowheart]

Also please give us an update on your other thread when you get a chance!

https://www.goodreads.com/topic/show/...

[Ayshe]

Under Town maybe?

[Tab]

How many years ago did you read this?
Part of a series?
Time period/location story takes place?