Hello sir,
1. The property purchased in the name of your mother. Therefore, after her life time the property right shall firstly devolve upon his sons , daughters and husband.
2. When your mother was being an owner of the property, after her life time, your father would become one of her legal heir along with you, your brother and sisters. Therefore, he can execute a WILL to his undivided share of 1/5 share in the property and not to an entire property. Therefore, as per the WILL your son will get your father’s 1/5 share.
3. You may claim that your father had purchased the property in his wife name as benami. But the narration in the sale deed much necessary to decide the ownership of the property. If your brother and sister appose the WILL, you should prove before the court of law that the property was purchased from an earning of your father in his wife name as benami. But it is hard to prove before court of law.
Therefore your mother’s property will be shared to 1/5 share (son) , 1/5 share (son) , 1/5 share ( daughter), 1/5 share (daughter) and 1/5 share ( Beneficiary under WILL that mean your son) . Hope, I have clarified all your doubts.