So, you start out with two counties, and your brother-in-law is Duke of Brittany. But, he's childless and unmarried; you're married to his heir, the Duchess of Brittany. And you two already have a son of your own. So everything is set!
First, kill your brother-in-law. Then your wife inherits the Duchy, and you become Duke. Unfortunately, though, you don't have any powers of the Duke; just the title. And you can't ask your wife to give it to you, because the Ducal Laws of France don't allow it. So you're stuck. Luckily your son will inherit both your counties and the Dukedom, so at this point you have a couple of options. You can kill your wife, but that just makes your son the Duke, now your leige, and under the control of a Regent. Another option is to try to get yourself killed by going on hunts or whatnot. But eventually your son will take over the dynasty.
Unfortunately even though Brittany is independent, the Ducal Laws of France still apply, and you're stuck with Gavelkind Inheritance even though all your counties are Primogeniture. You can give all your counties to your son, but once the Duke dies the counties just get split up again. Not really sure what to do at that point.
This seems to be an unfortunate limitation of the game; France's ducal inheritence laws really shouldn't apply to Brittany but it seems like they have to.