Can unregistered family arrangement doc can be challenged?

Is binding of minor property is valid in unregistered family arrangement document? The family arrangement was made in year 1980 between 4 brothers (A,B,C,D) for landed properties brought by deceased grandfather 30 acres and 30 acres brought by my deceased uncle A (15 acres on his minor son SA1 and 15 acres on his name A) from their own self earned fund. Property was allocated as fallows: A got his minor son 15acres land, B and D got 15 acres each from grandfather 30 acres land and C got 15acres land belong to A. Using family arrangement doc B,C and D had changed their name in mutation records. "A" got two sons (SA1 and SA2), recently they got into property dispute. Son SA1 is not ready share his 15 acres land with his younger brother SA2 because he claims he is absolute owner of 15 acres land brought by his father before family arrangement in year 1980.Now son SA2 threatening me to give back his father property 15 acres (now my father owns it) as it belong to his father and he is claiming family arrangement was not bona fide. Also told other two uncles B and D to give back his father share of property. Otherwise he will file partition suit to get back his father share of property ie.15+7.5 acres=22.5acres. Can son SA2 can challenge family arrangement in a court of law? and get back his father share of property ie.22.5 acres?