Is a young man considered sinful if he obeys his mother and returns to propose to his maternal aunt's daughter after performing Istikhara prayer and refusing to complete the marriage, doing so to seek her (his mother's) رضا (pleasure/satisfaction), despite knowing that his grandmother will reject this marriage?

Filial piety and seeking parental approval are among the greatest acts of worship. If a Muslim can be dutiful to their parents and earn their approval, that is best. If you desire to marry a girl and can fulfill your mother's wish, then do so. Try to convince your father to agree, and seek help from people of virtue. If the father insists on refusing, then it is obligatory for you to obey your mother, prioritizing her right to filial piety. However, if you have no desire to marry the girl, you are not obligated to obey your mother, and you should try to convince her. Otherwise, there is no sin upon you for disagreeing with her, especially given your father's lack of desire. What one feels after performing Istikhara is not a definitive proof. Whoever performs Istikhara sincerely will only be met with good.