Validity of GPA for partial property when one of the brothers doesn't participate in it

An unregistered agreement of sale was made for sale of 5 acres of agricultural land out of 10 acres in a Sy.No. in 1986 between the pattadars ( 4 brothers held it jointly with no partition done by metes & bounds, however, in revenue record 4 names entered with each shown as pattadar & possessor for equal share) and vendees. All 4 signed the sale agreement & all jointly acknowledged receipt of one fourth of sale consideration on the day of agreement. As per agreement second eldest brother ( local) was designated to receive the balance sale consideration, and it was paid in time. On the day of execution & registration of GPA deed in favour of Vendees' nominee, the youngest brother didn't turn up. Other 3 told the vendees that he is sick and they have his consent. An irrevocable GPA was executed by three brothers jointly. 18 years later, all four sold away the balance land of 5 acres and executed a sale deed jointly, declaring each is holding equal share in it. Queries: 1. Isn't this GPA valid ? 2.Can the brother, who didn't participate in GPA, question today it's validity, having been silent all throughthrough ?