There was a bug where you could only perform the joint attack with the Boglan forces against the rebels(or maybe betray them instead) if you had sent Lady Sessanah's sister to Boglan at some point. So, people who let her go find asylum somewhere else instead of seizing her and then negotiating her safe return weren't getting the right sequence of events after talking about the commercial treaty.
It should be fixed now.
Also, if you turn the "LET ME SKIP DAYS" option ON on the options menu you can now jump to the Boglan scene from the court menu.
I think I got it fixed now. (I was able to betray them after changing the indentation on a line)
@Phoenix_Wolf In order to convince some of the rebels to side with you, you need >= 40 Social AND having killed their previous leader during the ambush.
Currently, the only way you can "turn" on her is by siding with the rebels when you and her husband go after them, but I think you have a good point there.
Still, the words of a spy might not be enough to start a full on war. Dullis and Boglan are old rivals, and Dullis has much to gain by sarding Boglan over. There's a high social check that opens up the option of you questioning the spy about it not being some kind of manipulation from his part. I think it even suggests that Dullis is behind the funding for the rebels.
The lore also constantly makes a point of Dullis being a hard place to reach and to conquer, the bane of many armies and invading forces over the years. The player's forces can only maneuver in it because they are being allowed to and they are being guided by the locals. Still, I think it would be a nice addition to the game to at least give the player the option to try to make some larger military action there.
Especially because, with the player's army busy there, SIelccia would have an easier time invading your homeland.
As it currently stands, the chain of events needed for that is:
Get attacked while going to Boglan
Talk about finding the Rebels with Lady Sessanah
You must not keep her sister from her(either returned her or let her go before or right now)
Then the night before the attack on the rebels you receive the letter and you decide what side do you want to pick.
It's very specific, though.