How to challenge will made under foul circumstances?

In Mumbai, my father-in-law's will was registered during a home visit by a registrar. He is now deceased, leaving behind two sisters, but only one was named as a beneficiary. She is pursuing probate in court. After reviewing the registration video, we saw that the testator didn't speak or appear aware while signing, and was guided by the beneficiary and an acquaintance. Shockingly, the registrar didn’t question him and physically assisted in imprinting his thumb. Witnesses attested. Can we pursue criminal charges for forgery and fraud, and how can the other sister seek justice?