When making a will, it is perfectly acceptable to be influenced by people - but where this influence becomes undue, then the will might be challenged.
The question of what amounts to undue influence rather than simply influence has often been considered by the Courts. For influence to become undue, it must involve coercion.
A case in which coercion was shown was Schomberg v Taylor  EWHC 2269 (Ch). The case concerned the will of Mrs Taylor. In 2005, she made a will leaving the bulk of her estate to her two stepsons, with whom she had a very good relationship. Her brother in law, Mr Peskin, had previously been named executor in her will but had been removed after asking for money from Mrs Taylor.
Following the death of her husband in 2008, she had been very vulnerable emotionally and physically - undergoing surgery and suffering from depression.
During that time, she had received numerous phone calls from Mr Peskin who 'persuaded' her to change her will and leave the bulk of her estate to his three children (to whom she was not close). He was suffering financial difficulties, and wanted to get some sort of security for his children in the future. It was reported that Mrs Taylor felt under a lot of pressure from him - bursting into tears during a conversation with a friend and saying that she could not cope and did not know what to do about her will.
Eventually, in order to get Mr Peskin off her back, Mrs Taylor agreed to make a new will leaving her estate to his children and very little to her stepsons. On her death, they challenged the later will and were successful. The Court found that Mrs Taylor had been extremely fragile after her husband's death and the persistent pressure from Mr Peskin amounted to undue influence. Mrs Taylor had changed her will only to stop him from continuing to harass her.
An example of where an argument of undue influence has not succeeded is the case of Brennan v Prior  EWHC 2867 (Ch) where it was found that the testators will had not been overborne, although he had had been persuaded to change his will.
The case concerned the estate of Francois Devillebichot. He had one daughter who was estranged from the rest of the family. He was not named on her birth certificate (there was a dispute over paternity which had to be resolved using DNA evidence) although they did have a relationship.
Mr Devillebichot had a history of illness, and was diagnosed with cancer in 2009. He had considered making a will on several occasions, but had not actually done it. His daughter's evidence was that he had intended to die intestate so that she would inherit his estate.
In 2011, his cancer was diagnosed as terminal and he and two of his four siblings discussed making a will. He was unable to communicate easily by that time, and so the siblings drafted the will. He was admitted to hospital, and the siblings and executors visited him and got him to sign the will. That will left his daughter £100,000 and left the remainder of the estate to his siblings.
His daughter challenged the will on the grounds that it was not properly executed, that the testator lacked capacity, that he did not know and approve of the contents of his will and that there was undue influence from the siblings. She did not succeed. In terms of the undue influence allegation, the Court found that she had not proved that there was coercion - pressure which overpowered the freedom of action of her father. There had been persuasion by his siblings, but this was not sufficient to amount to undue influence.
The question of whether there has been undue influence will depend heavily on the facts of the case - the characters of the testator and the influencer, the factors which might make the testator vulnerable to coercion and the amount of pressure placed on someone. This can make these cases difficult to predict, and the more evidence that can be gathered to support the case, the better.