What is the effect of Malala addressing the audience as her "brothers and sisters"?

Term of endearment making her audience feel as though she is part of her family. Also a feeling of familiarity which will make her audience more likely to listen and acknowledge what she has to say with an open mind.

The effect Malala garners by addressing her audience as "brothers and sisters" is a form of familiarity ... she speaks to her audience like they're her family. It's personal, and it's endearing.