Who is ... (you) sister?
Who is YOUR sister?
She is ...(we) mother
She is OUR mother
This is ... sister
This is HER sister
That is ... twin brother
That is HIS twin brother
That is ... backpack
That is HIS backpack
I don't like ... (I) shoes
I don't like MY shoes
Those are ...(they) books
Those are THEIR books
... ball is pink
ITS ball is pink
... mom is very beautiful
HER mom is very beautiful