https://www.wowhead.com/news=311970/...taliesin-eviteThe Shadowlands has been suffering for a while, and the Jailer has more allies beyond Sylvanas.
Sylvanas is an ally and partner of the Jailer, not his minion. If you think Sylvanas is his servant, you didn't understand her character at all. Sylvanas is someone who values her freedom above anything else. Shackling herself to a new master is the last thing she'd do. We saw that in WC3, when she refused to serve the Dreadlords. We see that now with the Jailer, as she refuses to be a mere minion of his.
You do realize that Sylvanas later on tried to kill Alleria at the Battle of Lordaeron, yes? Then sought to start a massacre at the Siege of Orgrimmar 2.0, which could've led to the deaths of both Alleria and Vereesa. Even if that had not been enough, she always intended to unleash both N'Zoth and the Scourge upon Azeroth. Therefore, it's easy to see why she'd think her sisters would die anyway in the coming wars she'd start.
I am referring to when she fired that poison arrow at Arthas. At that point she was already undead. She could've killed Arthas in an instant (Arthas himself even acknowledges this), yet she refused to do so. She took her sweet time monologuing and then told Arthas she'd be slowly torturing him. This is who Sylvanas is. She likes to play with her food, instead of just eating it right away. This is the same thing she did with her sisters. She could've killed them right on the spot. But since she knew they likely wouldn't have survived the coming conflicts (indeed, Alleria would've died at the Battle of Lordaeron if it wasn't for Jaina), why bother doing that?
Which is, you know, exactly what she tells her dark ranger. That she'll let them cling to life for a little while longer, because eventually they, along with the rest of the world, will serve Death. This is further reinforced later on in 8.2.5, when she repeats twice the iconic line "Nothing lasts", as well as Saurfang remembering her last words to Vol'jin "In the end, Death claims us all".
As I said, there's nothing deeper here.