Question : can my brother become king

[Ludo_ZPM]

If my brother marries the daughter of a king, when does he die does my brother become the new king? THx

[LostScout]

No. The title of king in Bannerlords is not inherited. You have to be a clan leader to be king and that requires a vote by the other clan leaders in a kingdom. If your brother marries the daughter of a king she would join your clan so he would not be eligible in an election.
If your brother marries a female clan leader he would join her clan. If she died he would have a good chance of becoming leader of her clan, I've seen it happen. As a clan leader he would then be eligible to be elected king if there is a vacancy, but the odds of that happening in game are really low.

[Clovis Sangrail]

If you die or retire, you can designate your brother as your heir. He becomes your main character and clan leader. As your main, he could found a faction, or join a faction, If he joins a faction, he could get elected king if something tragic should befall that factions king.

But in the sense that you are asking -- No.

[TruXurT]

Well you can role play this, but in this games kings are elected. Lords tend to elect player character, especially when you play well and rise your reputation with them. Now in order to be elected you need to be a head of a clan.

To make your brother a head of a clan, well you will have to get a bit genocidal... Find a clan in a kingdom that you want your brother to rule (or at least be a vassal) that is preferably small and has an old single female lady.

Marry your brother to that lady and kill off all the other members of that clan (including the lady) so your brother becomes the clan leader.

Then it is up to him to rise in ranks of that kingdom. I am not saying it will work, but at least he will have a shot.