It is very difficult to draw firm conclusions without knowing more about the situation; e.g.
Was child B: (1) repeating a joke without considering that GF might apply it to his own situation; (2) teasing GF but without realizing that this was something serious to him; (3) deliberately trying to ridicule GF or get a rise out of him?
Was the topic: (1) something that related to a past episode that was unpleasant but has been solved and is not deeply personal; (2) as above, but involves significant personal embarrassment to GF (e.g. he was the one who crashed an expensive car): (3) something which involved serious personal pain to GF - e.g. a health scare- or a personal characteristic about which he is very self-conscious?
Did the child repeat the offence because (1) they had three jokes to tell, and were unaware that they were causing offence; (2) family members gave hints after the first joke that the child should stop, but the child didn't pick up on them; (3) the child deliberately ignored instructions or gestures to stop?
Did the parents and brother sit open-mouthed because (1) they were aware of the connection to the GF's situation but the child wasn't; (2) they were picking up signals from the GF that the child wasn't noticing; (3) the child was being cheeky by any standards?
Is the GF's reaction mainly (1) hurt feelings at what he perceives as deliberate ridicule; (2) annoyance at the child not being respectful enough to an older person; (3) offence that the child and perhaps other family members are not recognizing his status as 'head of the family'?
Is the fact that he is including A in the punishment due (1) to the punishment involving a deprivation that is practically difficult to inflict on just one of the children; (2) to a suspicion that A was somehow involved to a degree - e.g. that A had told B about the embarrassing situation; (3) to a desire to force A and perhaps the parents to discipline B in GF's own preferred way?
Without knowing the answers to any of these, we cannot form a firm conclusion.