The transfer itself would be exempt from tax in both cases as it is gift by a relative.
But when the 20 L would be invested by the wife eg in a FD, the interest income will be clubbed in the hands of the husband.[64(1)(iv) Income from assets transferred directly or indirectly to the spouse without adequate consideration]
Even if the mother transfers the amount directly to son's wife,the income on investment of the amount would be taxable in mother's hand[64(1)(vi)Income from the assets transferred to son’s wife.]