Case on will

A will or testament is a legal declaration by which a person, the testator, names one or more persons to manage his/her estate and provides for the transfer of his property at death. Mostly deseased persons prepares will as their precaution to death.So it is valid. 

Dear sirs/madams, My mother brother was under treatment for the disease called Dementia. The diagnose &treatment certificate is with me. Whether the will made by patiant with Dementia disease is valid or not.