Its about how you are going to treat people. In western culture there is no concept of treating everyone as if they were family. Western culture is very self centred where it is me against them. My family versus your family.
But in eastern cultures everyone is aunty, uncle, bro, sis. It does not mean they are literally family but it is used to symbolise your moral duty to give that same respect and love to all of humanity as if they were one of your own. This symbolises all of humanity being one family where we care and feel others pain as if they were our own family. This humanity reasoning is the why everyone is addressed as if they were family.
But obviously you have to marry so we will all marry within the humanity family....if you know early on that you have another intention then you shouldn't call them bro/sis.....the words then are mangetar or hooneh wali/a.
So in a nutshell people use family names like aunty/uncle/bro/sis to show humantarian love.