Thank you for your additional comments and information. Your original question was how long your sister in law has to challenge the will, and I believe I answered that question.
I understand that you have additional queries, however, and will do my best to assist.
It seems that your sister in law is intending to challenge the will on the grounds that she believes your father lacked the required level of capacity.
Testamentary capacity is normally presumed unless evidence is put forward to cast doubt on this. Where your sister in law is able to present such evidence, the burden of proof would then shifts onto you to prove your father did have sufficient capacity. There is plenty of case law to evidence the fact that succeeding in a capacity challenge is not easy, principally because gathering evidence of lack of capacity can be quite difficult 'after the event'.
It may be helpful to understand the legal test to establish testamentary capacity. In short, in order to make a valid Will, a person must:
- Understand that they are making a Will;
- Understand the extent of their estate and what their assets are;
- Understand the claims which may be made against their estate; and
- Not be suffering from a mental disorder which affects their capacity.
Where a challenge is brought, medical expert evidence on that point will always be sought. The expert will prepare the report based on a review of the patient's medical records, but since this is all dealt with retrospectively, evidence of friends and family, the witnesses to the Will and the Will drafter will often be extremely relevant.
Should your sister in law bring a claim that you wish to defend, I would strongly recommend that you instruct a local solicitor to represent you in such an action. They will be able to talk you through your specific circumstances, and the best way of collating and presenting the necessary evidence to the court.
I hope that is helpful.