It's the renaissance, it's Italy, it must be time for a war!
Time for Talena to do her Joan of Arc act again. I'd say St Talena of Parma has a nice ring to it, and she's definitely on the side of the angels this time.
From a gameplay point of view this looks to be a miscalculation by Milan. As long as they don't drag in any powerful allies and Modena stays neutral you should be able to take their two disconnected southern provinces.