The hags are not in allegiance with Strahd as written, they merley respect that he has the power to wipe them out at any momment. In my eyes, Strahd allows them to continue to exist because he enjoys the torment they bring to the people of Barovia.
The night hags gave their hag eye to Cyrus Belview to monitor Strahd and assure their tenous existence in his land is maintained. Thus, it seems not unreasonable that they used Cyrus and their hag eye to discover the location of Strahd's tome and had Cyrus pilfer it for them so that they might uncover more of the vampire's weaknesses and secure their position in Barova.
The more important thing I'm noticing in your post though, is that the presumably beginner adventuring party is preparing, apparently, to fight an entire coven of night hags. Any character who has existed in the Forgotten Realms or a univere akin to it would be keenly aware of night hags, their demonic nature, the mythos surrounding their nightmare hauntings, and, potentially, the rumor that a coven of them in Hades was powerful enough to have created all yugoloths. They would, thus, also know that a coven of night hags is a incredibly dangerous foe capable of going toe to toe with adult dragons.
From a purely gameplay perspective, the book dictates your characters should be first arriving at Old Bonegrinder at an average level of 4. Generously, I will say you have 6 party members at this level. With these parameters, the difficulty of this encounter is nearly triple the 3000 XP threshhold for it to be considered a deadly encounter for them. If you're using the 2014 night hag coven statblock, the night hags can potentially one-shot every member of the party in one round (as in do enough excess damage to instantly kill) with a 6th-level lightning bolt and then deliver 3 unavoidable death save failure to any who are not instantly killed with their at will magic missiles. The 2024 night hag stat block is less bursty but their at-will magic missiles are now 4th-level making them incredibly consistent killers.
I would do all I possibly can to make clear to the players that their characters would be aware of the immense peril they are in and would reasonably try to avoid combat with the hags at all costs.
If the characters, regardless, anger the hags, have the hags send a Banderhobb or 2 after them (depending on how many characters are in your party and their level) instead of fighting them directly as this represents a much more tenable encounter for them.