The Potter Twins and the Philosophers Stone

Harry and Cassie Potter are an ordinary set of twins who lives in a cupboard under the stairs and in the attic at their Aunt and Uncle's house. They live a very unremarkable life with only the odd hiccup (like hair growing back overnight!) to cause them much to think about. That is until an owl shows up with a letter each addressed to both Harry and Cassie, what will happen? ;)