The Ichadhari snake takes the form of Ganga to continue it's vengeance and it's up to Janaki's spirit to save the family from the curse of the snake.
The Ichadhari snake goes into Ganga after asking her (eventhough Ganga said no) to continue it's vengeance and kill the family. So it's up to Janaki's spirit to save the family from the curse of the snake.