Having a fun game with DVIP and a couple of mods, including the female inheritance.
Started as Count of Loon (that little county squeezed between the Rhein and "Schelde" rivers). Managed to become Duke of Luxemburg, and going at war with my biggest contender, the Duke of Lower Lorraine. Having a good war, died in the meantime, got inherited by my nephew, and ready to sign a total victory-peace, when I suddenly noticed the current Duchess of Lower Lorraine is... my aunt! That is, the wife of the former Duke.
Going back through the autosavegames, I saw that the Duke of Lower Lorraine indeed was the brother of the wife of the Luxemburgian Duke (myself, that is) who started the war... And he died earlier than I did, so it became actually a war of the spouses
And then I remembered why that is... I married her, because she was the heir of the childless Duke, and so I would inherit Lower Lorraine. But alas, she remained childless as well, so I had to refer to warfare. But I completely forgot that
o Well, it was already 20 years ago. You wouldn't remember either 
Started as Count of Loon (that little county squeezed between the Rhein and "Schelde" rivers). Managed to become Duke of Luxemburg, and going at war with my biggest contender, the Duke of Lower Lorraine. Having a good war, died in the meantime, got inherited by my nephew, and ready to sign a total victory-peace, when I suddenly noticed the current Duchess of Lower Lorraine is... my aunt! That is, the wife of the former Duke.
Going back through the autosavegames, I saw that the Duke of Lower Lorraine indeed was the brother of the wife of the Luxemburgian Duke (myself, that is) who started the war... And he died earlier than I did, so it became actually a war of the spouses
And then I remembered why that is... I married her, because she was the heir of the childless Duke, and so I would inherit Lower Lorraine. But alas, she remained childless as well, so I had to refer to warfare. But I completely forgot that