Did you have dual dialogues disabled in the options? If not, there should have been a dialog when Leandra's health hit zero about whether or not to do the soul forge (assuming you have the ritual, which presumably you did).
That was a bug; the sisters should have continued following into the final fight.
I would guess Icara showed up because the sisters were not involved in the fight. Normally, Leandra will take control and turn on the party, and you will either have to kill them or keep them CCed for a few turns until Icara can regain control.