Harry Potter and the Parselmouth Sisters

Mr and Mrs Granger have long come to expect strange happenings from their daughters, Hermione(biological) and Rosalina (adopted), but when Professor McGonagall comes to their home after letters from an odd school called 'Hogwarts' arrive for their two daughters, their lives take a turn for the fantastic. Author's Note: There are some adult bits (chapters titled NSFW) so be warned