Is a son's refusal to lend money to his father considered disobedience if the father requests the money for a real estate investment, given that the son needs the amount to build his own home and does not want future problems with his siblings?

Obeying one's father is obligatory unless it involves disobedience to Allah. Disobeying him is a form of undutifulness ( 'uquq ) and a major sin. The child and his wealth belong to his father, therefore, one must obey the father in what he requests as much as possible, for this is one of the greatest acts of devotion to Allah and a cause for success. The father's supplication is answered and not rejected.