he is my sister's favourite star为什么要's?

my sister's favourite season is winter连成句 My sister's hobby is collecting stamps.为什么sister后面要加's,为什么co His sister is my mother.He's my( ) He is a teacher of my sister和He is a teacher of my sister's my brother's sister is my(s ). My mother's s.is my sister. my favourite food is eggs .为什么egg加s了? 请问“My sister's dream is_____(become) a famous star. 说明为什么选This is my _____ dictionary.A sister's B sister,Mary's father's,favourite,fruit,my,apples,is my favourite subject is p.s.作文 ( )《read》books is my father's favourite