If your wife and her sisters wish to challenge the will they should see a solicitor as soon after the grandfather's death as possible as there is a strict time during which this can be done. It was usually taken to be a year and a day from probate being granted but I am not certain whether this has now altered to a year and a day from the death.
A will may be challenged
a. where there is a death without leaving a Will resulting in members of the family (particularly with extended families) being excluded from benefiting from the estate.
b. if the Will results in a distribution of the estate contrary to the apparent wishes of the deceased, a claim may be possible against the professional adviser who prepared the Will.
c. there is evidence that the deceased�s Will was made under duress, or without the appropriate capacity to make it.
In your wife's case (c) is the most likely reason but she would have to be able to make a case against her brother for exerting undue influence over the old man and a lot would depend on his mental state at the time the change was made;.