Summoned to the Harper Hold

There're cases in which Jaheira's forced absences during this quest break her romance, to limit the probabilities of encountering this bug, do not save & load game while she's away. Summoned to the Harper Hold is one of the two companion quests for Jaheira besides Save Jaheira from Baron Ployer's Curse in Shadows of Amn campaign. To active it, players must complete Bring the Poisoned Man to his Friends, then Find out What Happened to Montaron quests priorly. The quest has three additional names depending on different stages, listed below as headers.

Jaheira renounces her Harper status
Right after Xzar is killed, Meronia appears to inform Jaheira that her presence is needed by the Harpers (if you are in a romance with Jaheira, Meronia will only appear after certain romance stages).

Sleep somewhere to make Jaheira return, this time you'll accompany her to the Harpers Hold in Docks, where you'll be questioned by Galvarey, no matter what you say, he sees what he wants to see, soon it becomes obvious that he means you harm, stand with Jaheira and kill the twisted Galvarey and his fellow harpers. Note the Ring of Wizardry as a loot.

After four days passed, and probably some banters too, Reviane appears to fight you, beware if she appears in civilized areas as you don't want the cross fire to aggro innocents. Another three days passed, Dermin Courtierdale, another old acquaintances of Jaheira's, appears to tell that he's sent to kill her, their talk ends up and Jaheira renounces her standing as a Harper, Dermin leaves for now.

The return to Harper Hold
Then later Dermin returns and warn that Jaheira's death has been arranged unless she goes with him, which she agrees to think about, no more. Dermin leaves again.

The next time you sleep in the wilderness, Jaheira will left the party. Soon a strange Mage named Terminsel arrives and gives you a note explaining why she has left, urging you not to follow her, plus her destination - the Harper Hold in Docks, again. Return to the Harper Hold, deal with a group of mercenaries on the ground floor, go upstairs to welcome Jaheira back to team, kill a new group of mercenaries that arrive downstairs and get out.

The Harper plot revealed
About two weeks or so Dermin will appear with his goons, Jaheira and him have it out with a war of words, then the battle begins.

After the killing, Jaheira seems to be quite upset about this recent turn of events.

A long, long time (real time, so you can't just Rest to get this) has passed since the last event, Terminsel shows up again and asks Jaheira a series questions, Jaheira's firm answer leads to: Oh, and Terminsel? His name is an anagram for... Elminster. Impatient? Force Terminsel to show up via console:
 * 100,000 xp for protagonist + Harper Pin, if you're romancing her and have least 10 reputation
 * 100,000 xp for protagonist if you're romancing her and reputation is less than 10
 * Harper Pin, if you're not romancing her and have least 15 reputation
 * Nothing, if you're not romancing her and reputation is less than 15

CLUAConsole:SetGlobal("TerminselSpawn","GLOBAL",5) CLUAConsole:CreateCreature("jatermin")

Sources & references

 * Baldur's Gate II Walkthrough by Dan Simpson