Nope, we have confirmation that there are at least 3 Hives on Atoma, and that the Sister City isn’t inside of Tertium.
As for the main post.
I quite like Sisters, though I think they should be introduced from minor/local orders and be rejects, either being picked up on route to specialised Sister facilities or sent to Grendyl purposefully.
So no Power Armour and such, but being set on the acts of faith and such
And as for the Eldartide. As much as GW love to dunk on them, I don’t see that as likely, while having a named Eldar as an assignment target or as a Vocator could be much more appropriate to the lore.
In the end I think that Chaos Waste will be set in the Sister Hive, new factions don’t really need to be introduced through changing location, as they most likely would be local cults that revealed themselves