with his/her sister
Ella estaba jugando con su hermana en ese momento.
She was playing with her sister at that moment.
Él estaba casado con su hermana.
He was married to her sister.
Sin saberlo, se acostó con su hermana.
Without knowing it, he slept with his sister.
Tuvo que compartir una habitación con su hermana.
She had to share a room with her sister.
Puede que hayas confundido a Jane con su hermana.
You may have mistaken Jane for her sister.
Podría tocar el violín con su hermana, ¿pero si se lo quitamos también?
He could play the violin with his sister, but what if we take that away from him too?
— Ha vuelto (2015)
Iba al colegio con su hermana todas las mañanas desde que eran pequeñas.
She went to school with her sister every morning since they were little.
Tom compartía el dormitorio con su hermana menor Mary.
Tom shared the bedroom with his younger sister Mary.