AITA for trying to usurp my brother?

I (23M) feel pretty confident that I've always been the rightful heir to my father's throne, but my brother (35-52M), lets call him Arses, insists on being crowned King of Kings just because he's the eldest son. Our mother, the greatest queen we've had in generations, agrees with me and says that our father was still deliberating on his death bed. In fact, she's supported my claim for years and says that Arses was only ever a candidate to avoid more rebellions.

I understand that being the eldest usually makes people think they're their father's heirs, but it's really unreasonable IMO. Our father wasn't even king yet when Arses was born. they weren't even in line for the throne before Mother's friends helped them usurp the throne for themselves. If he wasn't born to royalty, it doesn't seem right that he should be king. There's an established precedent for this. Darius the Great made Xerxes I his heir for exactly that reason, and I don't see why we should change the rules now.

Plus, Arses' wife and all of their children are fruit of a poisoned tree. Her family were almost all rebels, and I think her last brother is scheming against me. Mother understands this. She says they should all be executed so Arses can marry a better match whose family didn't murder our eldest sister, but Arses refuses. How can you be king if your wife is a traitor to the empire? That alone should make me the heir according to Mother.

Arses' wife doesn't even respect our traditions, either. She rides around on a palanquin **with open curtains!** In full view of lesser nobles and even peasants, she's got open curtains for everyone to see her like some sort of commoner, and Arses just lets her do this. I would never stand for that sort of thing. In fact, when some of our cousins broke court protocol with me, I cut off their hands like a responsible ruler.

Then there's the experience issue. Eldest son of the king for years, but nobody even knows what Arses does for the empire. Meanwhile I've been hard at work as a Karanos, military viceroy for all of the Anatolian satrapies. I reclaimed our lost territories from the Greeks. I made the alliance with Sparta. I got the ineffective satraps back in line around a common cause, and they all support me up here. The only one who doesn't is Arses' traitorous brother in law, and that's because his whole family are natural traitors according to Mother.

But when I tried to point all of this out and take Arses place at the coronation, everyone freaked out, and I think I only escaped execution because Mother was there. Personally, I think I should hire 10k or so mercenaries and try again, but what do you think? AITA?