What does it take to have a person declared mentally incompetent?

my ex-mother-in-law has had a stroke and her children want to deem her mentally incompetent i dont want this to happen and i want to know what they have to prove so i can prepare one way or the other

They will have to contact a Medical Doctor who in turn will have to evaluate your ex mother in law and render his professional opinion as to whether she is able to make decisions for herself or not. If the Doctor decides that she is not able to act on her own behalf, then legal papers have to be drawn up by an Attorney to show that some family member has "Power of Attorney" over your ex mother in laws affairs. Hope this helps you.

I am sorry to hear about your ex-mother in law.
Basically, when someone is declared mentally incompetent, it means that they can no longer care for themselves with respects to personal care, finances, etc. I've included a link from an article I found that goes into more detail about the legalities of declaring someone mentally incompetent.
I hope that this helps. Wish you the best of luck.

A certified psychologist needs to determine this. Get one to prove she is fine....or not.