I have been playing my first Ironman game with Tribal and I started with Elective Gavelkind succession. After two character deaths so far I have noticed that my brothers become independent and I get a claim on their Duchy. After pressing the claim and winning the war though, I get the duchy but not the lower titles and the sibling remains independent. I wish I could play around for a bit and try the different options but I can't due to Ironman mode. Which casus belli should I use to make sure that my I can get the lands back?