The Mayor of London was bound by parliamentary protocol to get approval from universities minister Jo Johnson before he could contribute to a debate.
"Will my honourable friend allow me?" the Tory MP for Uxbridge and Ruislip asked, as his sibling was speaking.
Laughing, Jo Johnson, who is MP for Orpington, obliged, and later thanked his brother for his intervention.
Boris said: "Will my honourable brother and friend allow me on that point about diversity of students, is it not absolutely vital to maintain this country's high profile in those vital markets particularly India, where we've seen a sad falling away of student numbers coming to Britain?"
Jo, the junior brother by seven years but an MP for the past five years, replied to the recently-returned Boris: "I thank the honourable member for Uxbridge for his helpful intervention.
"Of course, we do encourage diversity of students, diversity of all groups that are under-represented - people from disadvantaged backgrounds, people from the most disadvantaged sections of society and we also welcome the diversity that comes from international students, and we hope to see numbers from the country he mentioned rise and stop declining in the years ahead."
The fraternal exchange came during a debate on mental health issues affecting students.
Labour's Kevan Jones, North Durham MP, could be heard labelling the intervention from Boris as "stupid", and later noted that he wished to make a "relevant point to the debate" when he rose to intervene.
Boris Johnson found himself having to ask his younger brother for permission to speak in the Commons on Thursday.