To "preside" is to be in a position of authority at a meeting or gathering. In this context it describes her as behaving as if she were an important princess attending the gathering, and depending on the situation it could either mean that she is very haughty or that she carries herself well. I'm sorry I don't know if this really helps!!

It means acting like a queen, overlooking a party, so being in charge of a party but doing no work.