Income clubbing in income tax

Are you referring to the family income because the family income has been defined as:

The total of the income figures reported for all individuals at the same address is called the household income. Persons in households who are related by blood, marriage or adoption constitute family households, and the sum of their incomes is referred to as family income.

The income your mother derived out of her own assets shall be her individual income and she would be liable to pay the income tax  if her income is coming under the tax bracket.

Your mother's personal income shall not be clubbed with your father's income and there is no reason for such merger.