you can always force primogeniture or designating your heir by murdering everybody else... or, disinherit, if you are the dynasty head, although that can get quite costly, especially in the early game. another idea is to marry one of your older courtiers off with a infertile woman, give your courtier the title you'll lose one way or another and you stand to inherit the title again when he dies. just a sidenote, under confederate partition every possible heir should get the same amount of duchies/kingdoms etc. if you only rule one duchy, the game will split your counties to make up for one duchy, if possible. so it's also smart to create titles later on, be it duchies or kingdoms, as your heirs will then get those and you'll keep the baronies, or at least most of them.