The troubles in the Holy Empire did not start because Aurelian's death without a clear heir was unanticipated--indeed, as the years rolled on and Aurelian failed to produce so much as a single bastard, the event became seen as increasingly likely. No, there were many plans of what to do when that happened, held by many people, and that--that was the problem, made worse by the situation of his prospective heirs, and the fact that the news didn't reach them all at the same time.
Aurelian's eldest sister, Julia had married the Count Oriflamme, an old family of excellent lineage. Unfortunately for her, her husband had angered Aurelian and been imprisoned, a situation he had the bad luck to die in. Julia and her children were all far from the seats of power, and only learned of what had happened after the war began.
Aurelian's second sister, Lydia, had married Ulrich, King of Tintagel. Aurelian had never trusted Ulrich, but even he had always had to be careful when dealing with the Holly Throne, one of the oldest traditions in the Lands of Light. His plan for dealing with Ulrich had been to keep him near him--the King of Tintagel had accompanied Aurelian on his campaign as an officer. Which meant that he was the first to learn of the Emperor's death--by many accounts, he actually witnessed it--and the first to declare himself Emperor. (Or more precisely, to name his infant son Emperor, with Ulrich acting as Imperial Regent.)
Aurelian's third sister, Augusta, had married Baldizar Cettes, a court favorite of Aurelian's at the time of the marriage. Aurelian's mind had changed afterwards--Baldizar's relatively low-birth was joined with a genuine charm and military skill that the Emperor quickly came to see as a threat AFTER marrying him to Augusta. Aurelian's way of dealing with this was to name Baldizar his Legate in the East, where Cettes spent his time trying to conquer, control and placate the hundred petty tribes and principalities of the region. Baldizar learnt of Aurelian's death at the same time that he learned Ulrich had declared himself Imperial Regent, and was marching on the capital with the remnants of Aurelian's great army. Baldizar's response was to declare himself Emperor--despite not even having had a child with Augusta yet--and rallying his own impressive army to meet with Ulrich's.
And that was when a fourth player entered the drama--Seisyll Pescheour of Corbenic...