If your mother is still alive, then the property will devolve to you, your siblings as well as your mother. All of you will have equal share in the property, as per Hindu Law.
Capital gains is computed by subtracting the indexed cost of acquisition from the sale consideration. The resultant amount will be the total capital gains and tax will be payable by each one of you at the rate of 20%, on your proportionate share.
From the sale consideration, you can reduce any expenses incurred for the sale, such as brokerage, etc.
Indexed Cost of Acquisition:
Indexed cost of acquisition will be the actual cost of acquisition as increased by the inflation index. The actual cost of acquisition is multiplied by the cost of inflation index for the year of sale and divided by the cost of inflation index for the year of purchase. The index for 2017-18 is 272.
Since it is an old property, you will require the market value of the property as on 1 April 2001 and the inflation index for 2001 is 100. So the market value of the property as on 1 April 2001, multiplied by 272 and divided by 100, will be the indexed cost of acquisition.
Also, in case you have incurred any expense after 2001 for the improvement of the property, then that can be indexed as well.
To claim exemption of capital gain, you can either:
1. Invest the amount of capital gain in a residential house in India. You can invest in a fully constructed property within 2 years, or in an under-constructed property within 3 years. You can also claim exemption if you had purchased a property 1 year before the date of sale.
2. Invest the amount of capital gain in Capital Gain Bonds for a period of 3 years. The interest arising out of such bonds will be taxable.You should invest within 6 months from the date of sale of property
Please note, each of you i.e. your mother and siblings and you will have to invest separately, based on your individual share in capital gains.
Your brother can claim exemption only if he does one of the above investments.