Tortoise by Candlelight by Nina Bawden

Fourteen-year-old Emmie Bean loves her family. But she worries about them too. Her mother is gone (no one will say where). Her father drinks instead of writing. Her younger brother Oliver has started stealing. And older sister Alice disappears on illicit dates. Then there is their isolated house's menagerie of birds and animals, including Mo the squirrel and Murgatroyd the tortoise, all of which require Emmie's love and attention. When the Sargents, a childless couple, move in next door, Emmie and the Beans find themselves drawn to this welcoming but aimless husband and wife. Emmie fears the darkness and chaos surrounding them all. But perhaps love burns bright.