Antoni is twenty-three when his father dies. 

In most of Cheliax, his father's lands would pass to his brother Galè, by virtue of the fact that he's two years Antoni's elder. Not so in Miravet. The longstanding law and custom of the Barony of Miravet (and by extension the Barony of Conesa; his family has controlled both since the civil war) is that all sons rule the barony. Not the way some places (weak places) do it, where each heir takes a fraction of their family's lands, but in the way of Miravet's ancient traditions, where all sons rule together. The custom well predates the Thrunes, but their great-grandfather fought on the Thrunes' side in the civil wars and secured as his primary concession the right for Miravet to maintain its inheritance law.

(And if some of his family's men die young, it's not as if that doesn't happen elsewhere too.)

Antoni is... willing... to share rule with Galè. (And with their younger brother Joan, he supposes, but Joan is a toddler so it's largely a theoretical question at the moment.) It is not his preference, certainly, but Galè isn't stupid enough for Antoni to have the slightest chance of assassinating him, and ruling together is better than losing everything in a failed attempt to take control by force.

The arrangement doesn't even last a full three years before Galè has him imprisoned in a tower on suspicion of treason. The charges are false, obviously, but he claims to have a witness, and it doesn't really matter how dubious his witness's testimony is when Antoni is imprisoned in a tower and Galè is not. His mother (sentimental fool that she is) objects; Galè imprisons her as well.

Antoni expects he will die there. The obvious thing to do, having imprisoned your brother on trumped-up charges, is to execute him. There is no reason to do anything else. It might lead to unrest, but so will imprisoning him; it might harm your Law, but it is not as if Galè had much to begin with. Antoni is not foolish enough to think there is any hope in petitioning Church or Crown for aid. If you are not strong enough to prevent your sibling from usurping your power, you were unworthy of holding it.

A day passes, then a week, then a month, and Antoni does not die.

He concludes, eventually, that his brother has deemed it safer to keep him out of the way without actually putting him to death. There's no use in speculating about his brother's motives; he speculates anyway. Perhaps his brother is worried that killing Antoni will somehow undermine his legitimacy. Perhaps his brother is worried that one of his sons will be tempted to hasten his inheritance. Perhaps his brother's witness died — but no, he would just manufacture another one, it can't be hard. 

Two years, two months, and eleven days after he was imprisoned, guards in the uniforms of his brother's staff arrive outside his room in the tower. For a moment, he thinks his brother has finally decided to execute him, but then they order him to his knees and demand he swear an oath of obedience to his brother and drag him outside and set him free and command him to leave the family lands and never return.

He stares up at the sky and drinks in the outdoor air and swears to himself that his brother will pay for what he has done.

Antoni is twenty-three when his father dies. 

In most of Cheliax, his father's lands would pass to his brother Galè, by virtue of the fact that he's two years Antoni's elder. Not so in Miravet. The longstanding law and custom of the Barony of Miravet (and by extension the Barony of Conesa; his family has controlled both since the civil war) is that all sons rule the barony. Not the way some places (weak places) do it, where each heir takes a fraction of their family's lands, but in the way of Miravet's ancient traditions, where all sons rule together. The custom well predates the Thrunes, but their great-grandfather fought on the Thrunes' side in the civil wars and secured as his primary concession the right for Miravet to maintain its inheritance law.

(And if some of his family's men die young, it's not as if that doesn't happen elsewhere too.)

Antoni is... willing... to share rule with Galè. (And with their younger brother Joan, he supposes, but Joan is a toddler so it's largely a theoretical question at the moment.) It is not his preference, certainly, but Galè isn't stupid enough for Antoni to have the slightest chance of assassinating him, and ruling together is better than losing everything in a failed attempt to take control by force.

The arrangement doesn't even last a full three years before Galè has him imprisoned in a tower on suspicion of treason. The charges are false, obviously, but he claims to have a witness, and it doesn't really matter how dubious his witness's testimony is when Antoni is imprisoned in a tower and Galè is not. His mother (sentimental fool that she is) objects; Galè imprisons her as well.

Antoni expects he will die there. The obvious thing to do, having imprisoned your brother on trumped-up charges, is to execute him. There is no reason to do anything else. It might lead to unrest, but so will imprisoning him; it might harm your Law, but it is not as if Galè had much to begin with. Antoni is not foolish enough to think there is any hope in petitioning Church or Crown for aid. If you are not strong enough to prevent your sibling from usurping your power, you were unworthy of holding it.

A day passes, then a week, then a month, and Antoni does not die.

He concludes, eventually, that his brother has deemed it safer to keep him out of the way without actually putting him to death. There's no use in speculating about his brother's motives; he speculates anyway. Perhaps his brother is worried that killing Antoni will somehow undermine his legitimacy. Perhaps his brother is worried that one of his sons will be tempted to hasten his inheritance. Perhaps his brother's witness died — but no, he would just manufacture another one, it can't be hard. 

Two years, two months, and eleven days after he was imprisoned, guards in the uniforms of his brother's staff arrive outside his room in the tower. For a moment, he thinks his brother has finally decided to execute him, but then they order him to his knees and demand he swear an oath of obedience to his brother and drag him outside and set him free and command him to leave the family lands and never return.

He stares up at the sky and drinks in the outdoor air and swears to himself that his brother will pay for what he has done.