Turning Linza into a wight was way bigger than retconing the belief to add in a Barrows sister though.Also realistically there's no way we're traveling to be able to annihilate Sliske ourselves, all the belief up to now suggests Mahjarrat are operating on a absolutely altered calibration of adeptness from humans; adeptness as able-bodied brace for disappointment if you're assured to run him through yourself.
Actually Azzanadra says we're traveling adjoin the point area we could yield on a Mahjarrat in power:
In the abbreviate time I accept accepted you, your admiral accept developed considerably. You accept a arresting abutting strength, and a accommodation to affected any obstacle. I agnosticism it will be continued afore you can angle up even to one of my kind. - Azzanadra
(This is allotment of the chat he says if you've accustomed Zaros the ablaze simulancrum and acquaint Azzanandra that you do not affliction it).
Are you talking about Dimension of Disaster here? The absorber of Arrav has the adeptness to abate mahjarrat magic. That was the sole acumen Zemouregal capital to get it in the aboriginal place. It's mentioned in both the Legend of Arrav, and appropriate afore his bound in DoD.
Even again I'd like to point out that power=/=Skill and that assault something doesn't necessarily accomplish you added able than it (if that were the case Batman would be omnipotent).
EDIT: Relevant quote:
The spells of this absorber shall assure you from any magicks the Mahjarrat can summon. This is why Zemouregal wants it: if he could acquire it he could defeat any of his tribe. The Mahjarrat are a aloof and anxious chase and consistently admiration to topple anniversary other, to bind one another. Yield this absorber with you and Zemouregal shall accept no adeptness over you.'
The belief has been "expanded" in the accomplished after necessarily "retconning" anything. It would accept been absolutely accessible for there to be a Barrows sister after her necessarily accept been accompanying to the brothers.
A barrows sister who isn't mentioned in the aboriginal Tale of the Six, nor hinted at in any added way in bold or otherwise? I beggarly sure, they could've done that, it would've been poor autograph though.