Is it permissible for the cousin to accept the agricultural land that his aunt sold to him through a nominal sale, while depriving the rest of the heirs of their right, knowing that he does not inherit from her upon his father's death, and her heirs (two sisters, a brother, and a husband) are alive?

If an aunt donates an agricultural land to her nephew while she is in good health, and he takes possession of it and disposes of it, then this is a valid and enforceable gift. The intention to disinherit heirs does not affect it, as donations completed during good health are considered to be from the principal of the estate, and there is no disagreement on this matter.