The word senile implies old age and loss of mental faculties. Neither of those necessarily precludes making a will or making a power of attorney. However, it is a question of degree and there does come a point when a person’s capacity is lacking to the extent that they may not do either or both. 

Accordingly, if your mother has made a new will or a power of attorney you could pursue a challenge to either or both on grounds of capacity. Each of those documents would need to be witnessed by an independent person (or two witnesses for the will) and the most common type of power of attorney (a “lasting power of attorney”) requires an independent “certificate provider” to confirm that the donor understands the nature and implications of the document and has agreed to it of their own free choice.

You should ask who the will witnesses and power of attorney certificate provider were. If the documents were drawn up by a solicitor, rather than homemade, they will be harder to challenge. I am not saying that just because I am solicitor but because the courts have found that to the case.

To challenge on capacity, you will require evidence and the optimum approach would be for you to somehow arrange for your mother to undergo a capacity assessment at this point. As the events you describe are recent, such an assessment would be almost contemporaneous. 

The legal test for capacity to make a will requires, in short, a person to understand that they are making a will, understand the extent of their estate, understand who may expect to inherit and weigh up their claims and not to have any mental impairment to making a will. Dementia could be such an impairment, but only “could”, and if the other factors could not be proved the challenge would fail.

Another way to challenge a will or power of attorney is “undue influence”, which in a challenge to a will amounts to “coercion”. This is generally considered harder to prove than lack of capacity because the primary witness (the person subject to coercion) is usually unavailable to give evidence, whereas a third party such as a doctor can give evidence as to capacity.

Another potential challenge lies in proving forgery or fraud. 

Broadly the same principles apply to challenging a power of attorney, the main routes being capacity, coercion or fraud.

But here’s the thing. While I personally enjoy the intellectual challenge of these types of case, the winners are always lawyers and rarely the family involved. Yes, justice and fairness are of vital importance, as is ensuring that a vulnerable person is not exploited. 

But here there may be a more pragmatic solution, which is to let your brother be executor and attorney. They are onerous roles and the key is to keep an eye on him. As a beneficiary of your mother’s will, when the time comes you can ask for evidence as to her assets and liabilities. 

If you are concerned about financial irregularities in relation to your brother acting as attorney, politely ask him to produce evidence of the transactions he is dealing with and if he refuses say (again politely) you are going to ask the Office of the Public Guardian’s safeguarding unit to investigate because you fear he is hiding something untoward. 

The OPG is the government agency that registers lasting powers of attorney. It also has a statutory safeguarding role. 

I do not envy you this situation and hope what I have said will help you navigate it in an informed and nuanced way. 

Ask a Lawyer is written by Gary Rycroft, solicitor at Joseph A Jones & Co, and published twice a month on Mondays. Email your questions to askalawyer@telegraph.co.uk