This does appear to be random. In the save I double checked, Icara and Leandra were soul forged, and brought to the final battle. That was using a random end game save, so I'm not sure what happened previously.
Had the sisters continued, either in the DK fight or (most often) the final one, Leandra would have taken over and turned against the party, and after several turns Icara would regain control, if you don't kill them in the meantime. It looks like that still happened in your game, even though they were not physically involved in the fight (or the game was just checking if the sisters were alive and Icara in control).
Reloading that save again and passing turns just healing Zandalor and Astarte, when Icara regained control, the sisters said "I have banished her, Source Hunters! Icara fights for you anew!", but then the Void Dragon killed her/them. I reloaded and did the same thing, and the sisters said "I have overtaken Leandra! Astarte, I stand by your side!", after which finishing the fight proceeded the same as killing the void dragon before the sisters turned back: Zandalor didn't know where the sisters were, and was going to search for Icara.
Reloading again and repeating, I got the 'banished' line, Astarte undid the soul forge at the end of the fight and Icara was in the Source Hunter Academy (as she would be if the soul forge ritual was not used).
I also had the sisters stop following once, but shortly after arriving, so reloaded.