The cuckoo sister - Vivien Alcock

"Since the day I found out about Emma, I seemed to have gone to the bad. I was rude. I told lies. I listened at doors and read other people's letters if they left them about. I was always losing things . . . watches, cameras, and silver bracelets. And whenever my mother reproached me, I screamed at her, 'Look who's talking? Who lost her own baby? Who lost my sister? Just because you wanted a new dress?'"

Convinced that her family's problems will end if only Emma is returned by the person who snatched her from her baby carriage, Kate longs for the older sister she never knew. But when a thin, spiky-haired stranger with hard eyes shows up with a letter claiming she's the long-lost sister, there's more trouble than ever. This "Emma" is certainly not the sister Kate imagined.