Any person — whether or not a relative — who has an interest in a Will can sue to contest it.
Reasons to contest a Will can include competency of the person signing the Will, whether the Will was the product of undue influence or fraud, and any other reasons that might make the Will invalid.
A Will contest must be started within 4 months of the start of the probate. After that time, Will contests are barred.
If the Will has already been admitted to probate, then the burden of proof rests on the person contesting the Will. If the Will was not probated, then the person claiming the Will is valid has the burden of proof. If a person contests a Will and wins, it is up to the court to decide whether to award costs to the contesting party. If the Will contestant fails to challenge the Will, the court can award both costs and attorney fees against the contestant, unless the court finds the contest was brought in good faith and with probable cause.