“Alzheimer’s, widowed stepmothers and estate crime are the three biggest causes of inheritance messes” according to estate and trust attorney Michael Hackard.  It is a high bar proving lack of capacity or undue influence.  Moreover, it is a low bar for one to have the capacity to sign or modify a Will (usually a higher capacity standard with a trust and POA)  It is important for attorneys to spot red flags and even turn clients away whose capacity is in question.  

https://www.thinkadvisor.com/2019/03/27/what-the-nastiest-celebrity-estate-battles-can-teach-advisors/