As children we get half our DNA from our mothers and half from our fathers. So I am half my dad and half my mum, all mixed up. THat means my sister also has half of each, but confusingly she could be made of the same or the other half of each of our mum or dad’s DNA. Make sense? If I had an identical twin though made by a fertilised egg splitting – we would both have the same DNA. THis is the only way two people can have the same DNA.