Maybe A MonsterBook - 1989
The monster referred to in this title turns out to be Sam's baby sister who has a habit of appearing at inconvenient times and spoiling older siblings' plans.
Mary and Sam are building a town with blocks on a rainy weekend. They are busy assembling a parade along the "monster wall" they have constructed when disaster strikes. First two ferocious tigers race through town (when someone has let in the cats) and the baby monster interferes.
"...Jill Creighton's love of language is as evident as artist Ruth Ohi's love of colour...This is a story that could have come straight from a child's playtime. Its underlying theme of sibling love would make it a great gift for young children with a new baby in the family." - Toronto Star