There are four grounds for contesting a will.
This argument says that the person who made the will wasn't capable of understanding what they were doing. They were not able to understand the effects of the will, the amount of property they were giving away or why they were leaving this property to a particular person.
A will might be overturned if a relationship of trust and confidence has been abused. Usually, this means that someone who was in a position to exert a strong influence over the deceased has substantially benefited from the will. It needs to be shown that this sort of relationship existed, and for there to be evidence that pressure was put on the person who made the will. The pressure must have been sufficient to overpower the will or freedom of the deceased.
Facility and circumvention
You might be able to have a will overturned if you can show that the person who made the will was weak and that someone else took advantage of them as a result. This might have been due to ill-health, old age, and that as a result, they were unable to prevent the other person from putting unfair pressure on them.
You may be able to challenge a will someone has benefited as a result of fraud.