Exchange of plot

Sir, My grandfather given one plot for example X to make house to his cousins about 70 years ago and in return he got another plot Y. but this was a verbal deed . After 20 years of exchange my father built a house on exchanged plot Y. these both houses are the only proof of exchange. Fortunately prices of plot Y shoot up very fast. Now due to greed my cousins gone to legal court for cancellation of deed.Please guide me what should I do, and how can i legalised this deed.