Unfortunately, as it stands, the new main residence allowance or main residence nil-rate band (MRNRB) won’t be applicable to family members that aren’t direct descendants.
When the new allowance came into force in April of 2017 it will initially extended a £100,000 allowance to every UK resident who left their main residence to one of their children (including adopted, foster or step children), their grandchildren or to their children in joint names with their respective spouses.
This means that by the 2020 tax year, when the main residence allowance has risen to £175,000, the average childless couple will be £350,000 worse off than couples with just a single child when it comes to inheritance tax. Any unused main residence allowance can also be passed between spouses, although this will also be redundant for childless couples.
For the time being, those who don’t have direct descendants to whom they can leave their main residence will be unable to make use of this allowance.
Although the inequity of this suggests that we might see the law amended at some point in the future, for now those unable to make use of the new nil-rate band will find themselves facing larger potential inheritance tax bills than their peers and will need to act accordingly.