I plan to sell some apartments that I own in New York State to my sister. The property has been in the family for over 40 years and there is no mortgage. She will be putting down a deposit for about a quarter of the selling price and I will be holding the paper for her. Neither of us live in NY state and so we would like to be able to do this transaction as easy as possible and to avoid legal fees.

Does anyone know what legal documents we will need to complete this sale?

Please, please please read abut 100 or the 10,000 posts about issues and problems, when selling property to family

Spend a extra 400 or 500 dollars and get an attorney to do the mortgage, that will include a schedule of payments and to do the need deed to file.
Do not do this yourself for your own protection.

Also make sure you know the foreclosure procedure if they don't pay, sorry but after doing these questions for enough years, we see this exact issue after the loan is done going bad time and time again. ** I know yes but not MY sister, guess how may say that also.