Who will fight in succession wars?

[Masquerouge]

I'm playing as France. I have my dynasty on the throne of Liege, but so does Spain and Brabant.
The current ruler of Liege is without heir, age 49, and the tooltip says that on his death a succession war will happen between me and... 2 years ago, Spain. Now, Brabant.

So, who will actually end up in the war? Brabant, Spain or both?

I have an alliance with Brabant, and with most of Brabant's allies too; how will they pick side? Random? Relations?
What will Austria do, will they side with Brabant as HRE, side with me as my ally, or stay neutral?

 

[Red John]

Whomever the tooltip declares will have a succession war, will be the only ones who fight, allies being the exception.

I believe what'll happen is, due to you being the 'original' dynasty, and therefore the dynasty head of Liege's dynasty, you'll get the PU. Brabant will then automatically declare war on you, regardless of RM/Alliance, which is stupid, but whatever.

It's all really dependent on whether or not they can call in their allies. Most will probably say no, if you're friendly, due to being an aggressive war.