Getting Rid of Marjorie — Betty Ren Wright

This fifth-grader is not too fond of her new step-grandmother -- “an affecting funny story” (Publishers Weekly).

When summer comes, Emily is looking forward to spending more time with Grandfather. Ever since Grandma Ellen's death, Emily has felt especially close to him. He's never too busy to listen to her, and he always understands her feelings.
 
But Emily's summer is unexpectedly ruined when Grandfather returns from a vacation with a new wife. Her name is Marjorie, and Emily hates her. There's no way Marjorie can replace Grandma Ellen, and she's certain to destroy Grandfather's happiness. So Emily decides to get rid of her. The jealousy and problems caused by Marjorie's arrival are refreshingly handled in the first novel by Betty Ren Wright.