2 solutions to the riddle

ראובן is obligated to honor his younger brother שמעון because of the Halacha in שלחן ערוך (YD. 240:22) "A person is obligated to honor his older brother According to the Ramban in Parshas וישלח 32:5 it seems, that because עשיו sold his בכורה to יעקב, therefore, עשיו the older brother would be required to honor his younger brother. The Ramban writes that יעקב insisted that עשיו be addressed as אדוני עשיו to show to עשיו as if the sale of the בכורה means nothing to him and he still treats him as the בכור by honoring him. יעקב addressing עשיו as אדוני is proof that יעקב does not consider the sale  legitimate otherwise he wouldn't call him אדוני עשיו. In truth, יעקב did consider it as a 100% legitimate sale. He wasn't Halachically required to honor עשיו but vice versa.     In a previous email I gave a different answer to the riddle The Hilchos Ketanos (2 :174) writes,Twin boys born in a leap year 1) in Adar 1 and the other in Adar2, being the younger brother becomes a Gadol (Bar Mitzvah) before his older brother*, the older brother is מחוייב to honor his younger brother".