may be dadi and nidhi read that letter and understand why shravan is with sanchi now because of their Cnight.....and they feel if shravan will come to know truth that he never had any relation with sanchi...physically he might go back to tara
and dreaded by their own imagination dadi adn nidhi decided to keep this secret between them to stop shravan going back to tara and in mean while they come up with all kinds of ideas to bring sanchi and shravan together.........
lol anything can happen......may be hitler end up reading letter lol.........hahah