Hi
If your mother-in-law and your wife, both don't want to make any investment in residential house property, it is advisable to invest the capital gain amount in 54EC eligible bonds.
Later, the money on maturity of bonds can be transferred to your wife. Also, your wife could be the nominee so that in case of any uncertainty before maturity of bonds, the transfer shall be done in your wife's name.
In this way, firstly there would be no levy of capital gain tax for your mother-in-law. Also, for your wife, it shall be an exempt income as gifts from relatives are exempt.
However, make sure capital gain is showed in your mother-in-law's ITR and exemption is claimed and gift is shown in your wife's ITR in year of receipt as exempt income.
Please note that investment in 54EC eligible bonds needs to be done within 6 months of sale of the property and the bonds shall be redeemable after 5 years.