Surely it all depends on the terms of the discretionary trust and whether they were authorised by the will. If the will provided for the niece to take at 18 but empowered the trustees to override that provision by creating a discretionary trust lasting longer, then she presumably remains just a discretionary beneficiary. If the discretionary trust terminated when she reached 18, or the trustees went further than authorised by the will, then it will be different.
Nicholas Le Poidevin, Q.C.
New Square Chambers