Nomination & Legal heir VS one of the Legal heirs

Recently, My Sister lost her husband (My brother in law) due to a sudden cardiac arrest. he was only 44 years old. They have a 17 year old daughter. My brother-in-law has a surviving mother who resides with her daughter (my brother-in-law's sister). The relationship with the mother has been tumultuous since there were some property related tension between my brother-in-law, his sister and his mother. My brother in law was kind enough to let them enjoy their father's properties (after his father's demise) in exchange for peace of mind. After my brother-in-law's sudden death, As per Hindu succession law, the class 1 legal heirs in this case are 1. His wife(my sister), 2. Their Daughter and 3. His Mother. My brother in law has taken quite a few insurance policies and holds a few savings bank accounts and Fixed deposits. He had named his wife as the nominee in all of them. My brother-in-law also holds a flat and a car in his name which are the only ones without any nomination. As such my sister being the first legal heir is also the nominee for Bank accounts, insurances, savings and fixed deposits. Though the flat and the car would bring all three legal heirs in the picture, It would be valuable information to know that my sister would stand to get 100% of all other payments. Can the deceased person's mother make a claim in all the nominee payments (my sister being both nominee and primary legal heir)? What would be the complications in trying to fight for 100% share of the flat and the car for my sister and her daughter, considering that my brother-in-law had given 100% share of their father's property to his mother and sister to enjoy?