The partition deed was executed by mutual consent and a favour distribution took place. Partition deed can be challenged only in case of fraud or coercion but nothing of that sort occurred. The partition deed is completely valid in the eyes of law and also no one had an undue advantage in the settlement as both lands were of equal value at that time.
Also, as you have mentioned the partition deed was executed in 1991, it cannot be challenged now as it is barred by the law of limitation. Article 65 of the Limitation Act, 1963 provides a time period of 12 years under which a deed of partition can be challenged on any grounds, now after more than 12 years have passed, it cannot be challenged in a court of law.
The courts do not entertain a case after the limitation period is over. The only other way to cancel the partition deed is by mutual agreement. Both the parties can cancel the partition deed if they want to and execute a new partition deed.
However, if the younger brother does not want to give up his share that he has improved in years, then he does not have to. He is not legally bound to cancel the existing partition deed or give up his share that he acquired fairly in a transaction that was mutually agreed by both the parties.
Reference: Article 65 of the Limitation Act, 1963