his/her/their aunt
La mujer que él pensó que era su tía era una extraña.
The woman he thought was his aunt was a stranger.
Her aunt has three cats.
She looks like her aunt.
La tarta de manzana de su tía era deliciosa, así que se sirvió otra ración.
His aunt’s apple pie was delicious, so he served himself another slice.
Le hice hablar por teléfono con su tía.
I arranged for him to speak to his aunt on the phone.
Él se quedó en la casa de su tía.
He stayed at his aunt’s house.
He/She stayed at his/her aunt’s house.