My mother was in London last year.
Your behaivour was so bad in class yesterday.
His house is next to mine.
Her presence is all I need.
My class is very big and its decorations are beautiful.

Our proyect didn't pass the test.
Their friends are not a good company to be with.
My sister isn't shy, she doesn't like to mingle.
She wasn't at the party yesterday with her sister.
His poem wasn't the best.
Si pero no es she el posesivo sino her, me equivoque al ponerlo en negrilla.
a ya muchas gracias
osea her wasn't?
La oracion está bien estructurada. El posesivo en esa oración es HER. La oración queda: She wasn't at the party yesterday with HER sister. Siendo HER el posesivo que es lo que necesitas.
ya gracias