Make Room for Mandy (Merlin Books) by Kathleen O'Farrell
it was just a wet and miserable kitten, but it was the very thing that 12-year-old Mandy needed. She smuggled it into her room in Aunt Edna's friendless little house - and there they stayed. But not for long!
"It'll have to go." said Aunt Edna when she discovered it. "Pennikins" did go - but so did Mandy, for they ran away together. The runaways didn't get far, but their escape meant a new life and home for Pennikins and, after many ups and downs, the most wonderful surprise n the world for Mandy.