I am not sure whether you are saying that the wishes of your Nan's will have not been met, or whether the way she divided her estate in her will was not appropriate.
Dealing with the first issue, since your mum died before your nan, any bequests made to your mum are shared equally between all of your mum's children (section 33 of the Wills Act 1837). It is the job of the executor (your aunt) to make sure this is done. If you believe it has not been done you can apply to the High Court to have the executor removed, but in the first instance, contact the Probate Registrar (to whom your aunt applied) for free advice. Your aunt had to apply for probate to be able to deal with your nan's estate.
If it is generally agreed by ALL the beneficiaries that the will is unfair in its distribution, it is possible to enter into a 'post-death' variation. But this does not seem to be the case here.
If it is just one party maintaining that the will was unfair in its distribution, it is possible to take the dispute to court. In the first instance you should take legal advice with a solicitor. One has to file a claim within 6 months of the Grant of Probate to the executor, and any claims are limited to the spouse of the deceased, a former spouse, a cohabiting partner, a child or stepchild of the deceased, or a person being maintained by the deceased. On that basis, you and your brothers and sisters would not seem to have a claim, and you may already be out of time. Hence the need to ask a solicitor.