Your brother would need diagnostic report that father is not mentally fit 

you can examine witness to prove that father was mentally fit 

The hospitalisation for kidney related illness has nothing to do with his mental health. 

If your father was mentally sane and the Will he made was attested by two witnesses then the Will shall be considered to be valid , therefore if you are confident that your father was mentally sound at the time of execution of Will deed,  you don't have to worry about the challenge made by your sibling in this regard. 

You defend your case with the assistance of a skilled lawyer. 

your question is - how can your brother prove that your father was not in a sound state of mind when he made the Will -

so why are you worrying?

whoever makes an allegation is liable to prove it

only your brother would know how he plans to challenge the Will on the ground that the testator was not in a sound state of mind while making his Will

how can lawyers here or elsewhere predict what is going on in your brother's mind and how he intends to prove his case?!

there is a specific provision in the Indian Succession Act which states that even if a person may be in an unsound state of mind for brief period, he can still make a Will at a time when his mental faculty is in place

in your case there is no question of unsoundness of mind at all

just because your father was suffering from kidney failure would not mean that he was also suffering from unsoundness of mind

so let your brother prove his case and you should focus on your case

you just have to prove to the court that the Will was made by your father as per law. that the Will was attested by 2 witnesses and that your father was in a sound state of mind when he made the Will

only if your brother succeeds in proving his case that your father was not having a sound state of mind while making the Will, would the onus shift on you to prove to the contrary 

It is for the challenger to establish the falsehood of will. Medically kidney failure does not affect mental capacity of the person executing a will.

- If your father was mentally fit to wiring the Will i.e. Doctor has not diagnosed him regarding mental problem , then even he was having kidney problem , then also he was fit for writing the Will

- Further, if that Will was written by your father in the presence of two witnesses , then that Will is valid and it cannot be declared invalid on the ground of fathers mental problem. 

- Hence, if your brother files a case against that Will , then you can produce the two witnesses before the court. 

The Will shall come into effect ONLY after the Testator's lifetime, and NOT earlier. So your brother cannot legally contest it now. Moreover the burden of proving his allegation shall rest squarely upon him. If you fear that he may do so later eventually, obtain your family doctor's certificate (dated prior to the date of the Will) on the state of soundness of the mental health of your father and keep it along with the Will.

Hi, WILL always prove through the winteness. Suppose if your brother has filed a suit challenging the WILL then one of the wintenss has to give evidence before the court that at the time of executing the WILL your father has sound and disposing state of mind.

He needs to prove the same with some medical evidence or acts prior to death

Dear client 

To challenge a will, your brother would need to prove that your father was not of sound mind (also referred to as testamentary capacity) at the time he executed the will. To do this, he would need to provide evidence such as medical records, witness testimony, or expert opinions showing that your father lacked the mental capacity to understand the nature and extent of his assets, the persons who would naturally benefit from his estate, and the consequences of his actions in executing the will.

It is also important to note that laws regarding the validity of a will vary by jurisdiction, so it would be helpful to seek the advice of a local attorney who can provide guidance specific to your situation