I had to DoW her in my current game just to protect all my allied City States.
If a CS is allied to you then it cannot be allied to Austria, and therefore she cannot marry them out of the game. Your CS were protected without the DOW. Venice, on the other hand, has no such limitation...
I must say that it is peculiar that the devs would introduce UA that degrades a core game element. I wish the base game made it easy to exclude opponents!