Is it possible to cancel a settlement deed without the concent of the beneficiary

My Parent in laws have 4 children - 2 sons and 2 daughters and my husband is the 3rd among them. 1. My father in law owns a residential flat and a empty plot. he has given dowry and borne all the marriage expenses of the daughters, hence he has given the residential flat to his elder son and the the plot to my husband (younger son). 2. the residential flat was purchased in my bro-in-law name but was completely funded by father in law. 3. the plot was bought by my father in law and he did settlement deed to my husband in 2003. 4. now that, father in law is bed ridden, my mother in law is asking for the plot and wants to sell it and settle it as 5 equal shares ( 4 siblings and 1 for herself ). 5. My concern is , mother in law doesnt want to sell the residential flat which is in my bro in law name as they are living there but wants to dispose only the plot. 6. since we objected for partial disposing and settling everyone as it does not qualify equal partitioning, mother in law is asking for the original documents of the plot from us. 7. my question is , is it safe to handover the documents to mother in law as we are unable to deny due to threats...is there any possibility of she / or other siblings to manipulate the settlement deed given to us without our concent? 8 Can settlement deed be cancelled by my father in law himself without my husband's consent? 9. Lastly please suggest if there is any way to safeguard our stands on the plot as we have invested some money on the plot with the thought it belongs to us.