I think Anivia would win it for Ashe, it'd come down to a big magical Rocky-esque fight between Lissandra and Anivia. Sejuani would fall first since she doesn't have any godlike entities on her side and she'd probably join Lissandra since her vision for the Freljord (and ultimately the world) would be close to Sejuani's.
I think it's more likely that Sejuani or Ashe will realize what Lissandra is then join forces with the other one in order to protect Freljord. Along the lines of "I don't like you. But I don't like her more. I can put aside my differences to help you for the greater good."
After all, remember that Sejuani isn't actually evil. She just doesn't like the idea of everyone putting on fancy dresses. She wants to join Freljord by conquering it, not by having everyone gather together for teatime and AA meetings.