Is a sale that was secretly concluded for a plot of agricultural land for a negligible sum considered valid, and are the farmers entitled to claim partnership in it or ownership of part of it, given their long-term cultivation of it?

The preponderant opinion is the permissibility and validity of agricultural sharecropping (Muzara'ah). It is among the permissible contracts that either party has the right to revoke at any time. If the owner revokes it before the crop is ready, the worker is entitled to a fair wage (Ujrat al-Mithl). The owner may sell his land whenever he wishes without informing the worker or granting him the right of pre-emption (Shuf'ah). Therefore, you do not have the right to demand your brother's participation in the land unless he consents to it. As for the option of rescission due to a significant disparity in price (Khiyar al-Ghabn) for the seller, the preponderant view is its establishment, especially if he was compelled to sell due to laws preventing the eviction of the worker. This option is a right of the owner. If the women consented to the previous sale, the sale is binding. Before their consent, they have the right to revoke it.