I've been thinking about it, and now I headcanon that the reason Armand suddenly pulled a 180 after being rude and dismissive towards Amalia in season 3 and the wake of their father's passing is because Master Joris encouraged him to see past his own feelings and bitterness and bury the hatchet with his sister.
No, I don't have solid proof, but I have speculations rooted in canon.
Right after Armand comforted Amalia, he revealed he'd been invited to an assembly as King of the Sadidas and how he'd hoped his sister would accompany him, effectively treating her as his second-in-command instead of the spoiled brat he'd been seeing her as until then.
And who's there, all but acting as the official mediator between the rulers?
That's right. Master Joris.
Given he's always played a pivotal role in the World of Twelve and his position as Bonta's ambassador means he has close ties with the world's monarchs (as evidenced by his presence during every single meeting Yugo and Nora had with the other rulers, except maybe Brakmar), then it wouldn't be far-fetched to believe he was the one in charge of summoning Armand to the assembly.
And given his own ties with Amalia by virtue of working together as allies since she was 13, then he could've noticed her absence in the palace. Which would then prompt Armand to probably make some sort of derisive comment about how she had yet to leave their father's tomb-tree's side despite not being there during his actual passing.
Because I have a soft spot for Joris, as he is legitimately one of the most trustworthy and level-headed characters around, I want to believe he probably said just the right thing to get Armand to realise the role he played in their relationship going sour and make him question if he really wishes to be on bad terms with the only family member he has left.
After all, Amalia is constantly away, fighting with everything she has just to survive. Does he really want for his sister to die thinking her own brother doesn't love her?