If Carelia is losing then Caergoth and Suville are losing, too. The war doesn't belong to Carelia; it's just on their border. Caergoth and Suville are not the twins that they used to be, either. Geography is likely to give them common interest in the future, but their old Abington history, not so much.
I don't think they're twins, I think they're opportunists. Don't get me wrong, what Carelia is doing is audacious and if successful will certainly shake things up a bit. But if she's not successful, I reckon she'll be made to pay a heavy price for her failed ambition. They're fighting for land as much as Carelia is and they'll get it one way or another.
