"Two legends fighting one epic."
The senior sister made her judgement: "Unwinnable."
If it were two Living Saint legends fighting a common epic, there would definitely be a chance to fight, but the epic on the other side is also a Living Saint.
With the same level, but a whole realm apart, the senior sister didn't think Morgan's side could win.
"Regardless of the outcome, if people get captured, it would be troublesome."
So the question arises, why Serafier and Skadi don't leave but instead stay in Morgan's Royal City of the South Sea Kingdom, to fight with Leia.
Because you can run from the monk, but not from the temple.
Morgan could leave, but his city, his Miracle Citizens, and all his mortal resources are all in the South Sea Kingdom, and that's hard to take with him.
What's more, packing up and running away, for the Miracle King, is a great disgrace.
Even if Morgan is concerned about his daughters' danger, Serafier and Skadi would absolutely not leave.