Ownership based on agreement of sale

Myself and my sister owned 50 percent of ownership in a land of 5 acres and the other 50 percent was with my maternal uncle’s wife. The land is not demarcated. Upon the death of my maternal uncle’s wife, myself and my sister were the legal heirs as per Hindu Law and the other 50 percent share was also transferred to us. We are the legal owners of the land presently and the same is reflected in the RTC as well. We have suddenly received a legal notice from a third party claiming that there was an agreement of sale entered between the third party and my maternal uncle’s wife in 2020 for selling her 50 percent share of the land at a cheap consideration to them. Since she has passed away now, the notice informs that we (myself and my sister who are now the legal owners of the subject 50 percent share of the land) are obligated to fulfill the terms of the agreement of sale (we don’t have a copy of the agreement even) and that we have to execute a sale deed in favor of the third party at the mentioned consideration failing which civil and criminal suit shall be initiated against us. I would like to know what legal course of action should be taken in this scenario by us. Thanks in advance.