Transfer security deposit from tenant to tenant question

[Dmitry Prosvi]

Hi! Enjoy reading the forum and now have my own question.

 In April this year my tenant decided to move out of the house due to personal reasons while on year long lease that ends this month. To save the situation he was upfront with me and said that he was going to allow his sister live in the place till the end of the lease and that they would make all the rent payments that they did. I was very busy with reno wo4k at another place and agreed to this arangement.
Now when the lease is almost over his sister is willing to stay and I mentioned that I would have to put her on the lease which is fine with her. I mentioned to both of them that I do require sec. deposit to sign the new lease and if they work it out between themselves I would just keep the current deposit that is his in place. But what paper form should I use to make it  legit? Im thinking to have former tenant sign the security deposit refund receipt saying he got it back and use it as a new security deposit in his sisters name. This is in Illinois.

Thanks