WILL made for second wife. How to safeguard my interest

My mother expired 3 years back. I have 3 younger brothers and my father re married again. From this lady, he has a 1 and a half year old son. She was a divorcee with a daughter under 18. My father has 2 flats and a shop in his name. He wants to give her 1 flat. My father has already made a WILL (un-registered) before the second marriage stating the above but he has mentioned nothing about the property distribution for the second wife in the WILL. What documentation should be made for the peaceful transfer of the property? How strong is the WILL which he has already made?