That reminds me of something I recently read.
Poodle and Doodle by Donna J Shepherd is an adorable picture book about a tiny poodle who is upset when her humans bring home a gigantic labradoodle named Scruffy. At first the dainty little dog is upset by the new dog's clumsiness and lack of manners, but eventually she realizes living with Scruffy has advantages.
The book is told in rhyme that's comfortable and not artificially forced, unlike some rhyming picture books I've seen. The colorful illustrations by Jack Foster are perfect portrayals of the cute and amusing story.
Poodle and Doodle would appeal to most young kids but it would be especially appropriate for those who love dogs and children coping with sibling rivalry.