He was rude, boorish and generally unpleasant.
Your sister's summary of the two of them being called the life of the party suggests two, not mutually exclusive possibilities:
1. They keep company with equally poorly mannered company; and/or
2. People commented that they were lively, and out of politeness did not add "and remarkably socially inept and inconsiderate!"
I agree that having some prepared comments in advance of seeing them in future would be wise.
I would be tempted to reply to any further of BIL's inappropriately communicated observations with: "and did you know that frequently, when confronted with poorly behaved people, better behaved people often do not criticize but simply deflect them? Knee jerk sympathy for jerks, or smoothing social interaction. Bean dip?"