How do you know what "su" means in a sentence since it has so many translations?
By context clues
Why do we need four different ways to say "our"?
So it can match the noun: masc/fem/sing/plural
Why aren't you able to precisely determine what this means in English: Su abuela
This is because the reader does not have a way of knowing which English definition of su is the one intended.
Translate to Spanish: Our little sister
Nuestra hermanita
translate to Spanish: My grandmother talks a lot.
Mi abuela habla mucho.
Translate to Spanish: Your sister is tall.
Tu hermana es alta.
Translate to Spanish: Her mother is in Spain.
Su madre está en España.
Translate to Spanish: Our aunt is 20 years old.
Nuestra tía tiene veinte años.
Translate to Spanish: Her aunt is my mother.
Su tía es mi madre.