Come to think about it, if Lissandra wanted to take the Freljord back for her Watchers, why didn't she attempt to kill Ashe and Sejuani while they were still children? Would have it made much quicker and easier to claim control of the Freljord? Alternatively, did she need them alive? Perhaps Ashe and Sejuani were needed to ensure that the Winter's Claw and Avarosan tribes would become unified and easier to manipulate?
Because no one really looks at Sejuani as a threat right now. She and her bandits really just go around breaking shit. Ashe, however, is already a pawn of Lissandra's. In the lore you find that Lissandra (not the Ice Witch) actually swore fealty to Ashe. Ashe thinks of Lissandra as an ally, exactly what Lissandra wants. Ashe isn't immortal, remember? Eventually, she'll die and Lissandra will assume power just as she did to the Ice Dervish.
She doesn't want to just "claim" it, she wants to rule it. She wants to present it all wrapped up in a bow for the Frozen Watchers when they return. Outright killing people doesn't make them like you. But taking the centuries to slowly corrupt them and make them work for you (like she did with the Frostguard) means she can make her enemies protect her.
The Ice Witch persona is basically reverse Batman. If she wants someone wiped off the map, she does it herself and no one suspects a thing.
Lissandra is f**king awesome.
Lissandra probably can't move freely around their respective tribes and simply kill their leaders,not to mention that if she killed those 2,then others would have taken their place and there is no forseeing if these others are better or worse for her plans.
In the end I'd say it isn't unlikely that lissandra already killed several leaders of the opposing tribes(she even has a taunt for ashe about killing so many queens),but others simply took their place instead.
She may have not known where Ashe and Sejuani were when they were kids, nor even noticing their presence (she doesnt have future sight like Garnet). Her initial plan was probably to wait until proper rulers present themselves and then manipulate herself into the Freljordian politics again.
Freljord has never been this unified as during the time of the 3 sisters.
Because they were just kids. How would she know that they would grow up into the powerful leaders they are?
because then we wouldn't have a story :V
