When a speaker refers to the audience as "brothers and sisters," they are 'breaking the ice' and making the audience feel closer to the speaker. It helps make the audience members and the speaker feel welcome.
We are sentient beings and must fulfill our purpose through interaction with our environment. Those decisions require thought, and thought is a response to an uncontrollable choice. I choose to sub to Pewdiepie.
It is an example of, theme. Theme would tell you how the mood is set. <em />It's not symbolism, because there was no object to describe that moment, or any past moment. Hope this helps!