Family Problems and Party ?

Ok This might take a while so I thank Who ever listens... So lets start with the basics a long time ago in a far away land my uncle had a child with his first wife then did not treat/recognize this daughter. He then moved on with his life and got happily married witch led to two more children. now his two youngest children dont know their step sister even exists. his two latest kids are around 12 and 13. So now we are having a party at my house and they are ALL invited because the long lost stepdaughter of my two little cousins is in college they are going to ask questions... Now the question is how do i answer these questions/ handle the situation while keeping the party together.