your sister; his/her sister
Beth está mais magra que a sua irmã agora.
Beth is thinner than her sister now.
Where does your sister live?
How is your sister?
He took his sister along.
O Tom vai escrever para a sua irmã.
Tom is going to write to his sister.
Mary não é tão ativa quanto a sua irmã.
Mary isn't as active as her sister.