Sin/sitt/sina is used together with an object which refers to the subject. Here, "bröder" is the subject of the sentence.
Below, "bröder" is the object and they "belong to" the subject.
Hon ringer sina bröder.
I understand, thanks very much :)
i am confused about the bröder,why not bröderna
It is "bröderna är långa" but "hennes bröder är långa" (compare to English (the brothers are tall and her brothers are tall").
Her brother is L O N G
Why can't you use stora?
That means big and not tall.