Here’s a fun bit of symmetry: Of the four French titles competing for this year’s Palme d’Or, the first to screen was “Brother and Sister” and the last was “Mother and Son.” (Presumably daughters and grandparents will get their due next year.) Of the two, “Mother and Son” director Léonor Serraille bests her colleague Arnaud Desplechin in the family-saga sweepstakes, delivering a decade-spanning immigration drama that plays on the most intimate of registers.The film closed out the Cannes competition on Friday, providing it an auspicious berth. This year’s jury will go into deliberations with actress Annabelle Lengronne fresh in mind; should the actress win, she won’t have far to travel.She isn’t entirely the lead, as the triptych follows a Franco-Ivorian family in chapters dedicated to each member.