Liabilty of income tax on whom ?

A Person has 5 sons. 5th son is mental challenged& unsound mind. Father purchased 11 Assests. out of the assests he registered 8 assests in the name of the 4 sons directly in their names i.e..,2 for each son at the time of purchase. 1 asset was sold by the father for which income tax on capital gain was not paid by him. Before the death of the Father, father written a will that he already given 4/5th of the property to 4 Sons and the remaining 2 assets are now transferred to the 5th son (who is unsound Mind) by 1st son as a guardian. After the death of the father the income tax department issued a Notice to all the 5 sons for the liability to pay the income tax liability on asset sold by the father which is Rs.1 crore to each son. The 4 sons contented that they are not liable for income tax payment because the demise is on the last son. Who is liable for the income tax liability ? Pls refer with a case law in this regard.