I think it could have mild innate armor piercing, but not great damage - maybe if fully charged, but it could have a very long charge time to compensate, and then have a slight fire AoE.
So its niche would be that you can use it to plink at armored targets or spam at unarmored ones for kills - rather like the swift bow - or charge for a nice DoT and AoE effect/stun. While this would make it similar to Swift Bow in basic and Hagbane in charged, it would be balanced out by the overheat mechanic - it’s not the bow itself overheating, it’s Kerillian using a magical item to channel the ambient winds of magic that is becoming overcome. It IS a magical fire after all, but nowhere in the lore does it state that the Sisters of Averlorn are trained magic users - I think since all elves have some inherent magical talent (including Kerillian, since she talks about having prophetic dreams in the first game), I don’t think it would be too much of a stretch.
This overheat would prevent you from being able to spam down hordes, because you would quickly overheat, and she has no way of quickly getting rid of that. So this bow’s niche would be like the Handmaiden’s; providing good and useful support fire, without being too good at any one thing. And since she lacks ammo recovery, she’ll always have it as an option, so long as she hasn’t overused it.
I’d keep it restricted to Handmaiden, however, since the Sisters of Averlorn/presumably Handmaidens are the only ones known to use this bow, and they’re High Elf units.
I didn’t think about the attack whips from the Sisters of Slaughter, haha. That’s a crazy idea that could be fun.