The RNRB available to her estate will be the lower of the value of her Qualifying Residential Interest (the value of her 15% share) and the default allowance on her death (eg RNRB of £125,000 in 2018/19) as long as it is inherited by lineal descendants. This excludes discretionary trusts.
Malcolm is quite right, although the RNRB can apply to a GWROB, it would only do so if the gift had been to lineal descendants-not to a discretionary trust