The Supreme Court observed that tenants cannot claim claim adverse possession against their landlords, since their possession is permissive in nature.
However, under the Limitation Act 1963, a tenant can potentially assert ownership of a house if the property owner fails to assert their ownership for 12 years and if the tenant continues to occupy the property for those 12 years. This is referred to as adverse possession of property. Ownership claims by tenants through adverse possession occur when the lease expires or when the landlord breaches clauses in the rental agreement regarding rent payment.
Now on the basis of your continuous occupation of the property by your father and on that basis you have inheritance rights for tenancy, you can continue to occupy the property, your cousin grandfather, if not having any rental agreement and the original owner is reported to be missing, then the proposed case by your cousin grandfather also may not be maintainable.
However it is not advisable that you file a suit for claiming title operating the law of adverse possession since one of the legal heirs of the original owner has already issued a legal notice to you to vacate the premises.