You do get a harper pin once Jaheira's quest is complete, but I don't know what triggers it. The first time I played, I didn't get one (Termisel didn't show up), but I did the second time. The first time, I killed all the harpers who attacked me, the second, I chose not to fight at the first encounter, and there were no further fights apart from killing Dermin. Maybe that's the difference?
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