Oh my. Sound like Castila and Austria has a common rival (most likely France).
You are right, they are both rivals to France.
I've reached 1514 so far in this game and Europe starts to look... interesting, to say the least.
England lost Wales and all of Northumbria (safe for Yorkshire) to Scotland and its French cores + Kent to France, but still owns the landlocked province of Armagnac.
Castile got Labourd and Bordeaux.
Austria annexed the eastern half of Bavaria.
Bohemia took over half of Hungary, with some smaller parts going to Poland, Serbia, Bosnia and the Ottomans.
Thomond is one province away from forming Ireland.
Byzantium exists as an OPM in Morea.