Contesting a will or inheritance decision in India involves specific legal grounds and processes. To successfully challenge a will, an interested party must establish one or more of the following grounds:

• Lack of Testamentary Capacity: The testator must have had the mental capacity to understand the nature of the will and its effects at the time it was made. Evidence of mental illness, cognitive impairment or lack of understanding can support this claim​​.
• Undue Influence or Coercion: This requires proving that the will does not reflect the testator’s true intentions due to pressure from another party​.
• Fraud or Forgery: A will can be contested if it is proven to be forged or if the testator was deceived into signing a document they did not intend to be their will. 
• Lack of Due Execution: For a will to be valid, it must be properly executed according to legal requirements, including being signed by the testator and witnessed by two individuals. Failure to meet these criteria can render the will invalid​​.
• Lack of Knowledge and Approval: The testator must be aware of and approve the contents of the will. If it can be demonstrated that the testator did not understand or was not informed about the contents, the will can be contested​​.

To contest a will, an interested party must file a petition in the probate court. This involves presenting evidence and possibly witnesses to support the claim. Given the complexities of these cases, it is highly recommended to consult with a real estate attorney in India. Seeking the expertise of a real estate attorney in India ensures that the procedural and substantive aspects of the case are managed properly, increasing the chances of a successful challenge.