Firstly, we would check that the will was validly executed because if it was not, it would not take effect. The law is quite clear about the steps that need to be taken to validly execute a will.
If it was, we would look at the circumstances of the will being prepared and if it was done by a solicitor, we would invite the solicitor to give their comments on how they came to receive their instructions and any concerns that they may have had about the capacity of the testator. It may also be necessary to obtain your mum’s medical records.
As you can imagine these matters are all very fact-specific and vary greatly from matter to matter. We would therefore need to look at the whole matter and advise you on whether a valid challenge can be made.