Name change in rent agreement

I own a flat which I have rented to a gentleman. He claimed it was for use of his family and sister. Gave me half the deposit upfront and gave me a pdc for the rest. Paid the first months rent upfront before agreement registration. When it was time for deposit pdc to be deposited he claimed sister was unwell and he did not have money to clear the same. When the second month rent pdc was due he asked not to deposit the same. His request: Stay on with half the deposit and lower rent. Turned it down. Met with both sister and brother. Brothers family had moved out. Sister said she wanted to live on and said she would clear all dues immediately. I agreed but only if the agremt would be done in her name. She agreed but requested that this change be done after a month. Now brother is chasing me to get the old agreement cancelled. Talking legal notice.sister has not signed the new one. Has cleared dues and pays rent. Whats the legal view on agreement in name of person other than one staying?