"Theseus, I don't quite understand why this is necessary-"
"Newt, this is important. I know Dumbledore doesn't trust me the way he trusts you, but I need to speak with him."
Newt fiddled with his hands, still uneasy when presenting himself in front of his brother. Usually, Theseus' gaze would be more affectionate when his visits are to congratulate Newt on whatever beastly mission he's down; or stern if Newt's almost got himself mauled by some XXXXX beast or got tangled up in political trouble. But now, it's neither. It's demanding - something he rarely uses with his family, especially his beloved brother. But Newt knows why Theseus is acting precisely the way he is:
Grindelwald's been around for too long. Far too long, considering their blood pact was broken years ago.
And that day, when Grindelwald escaped Bhutan, the professor had promised Theseus he'd track him now. But now it's been over five years, and Grindelwald and his Alliance are still lurking around the world, spreading their anti-Muggle sentiments across the globe. Theseus was an understanding man; as Head Auror, he's had to deal with some pretty nasty wizards and witches, even if it didn't result in magical duels. But he was also a man of his word and believed promises should always be kept. Newt knows that the fact Dumbledore's still in the safety of Hogwarts, as the world around them burns to ashes, has been bothering him.
"Maybe he has a plan, Theseus," Newt tries to reason. Of the two Scamander brothers, he's always been the more considerate and caring one. There's a reason why one's a creature carer and the other's a Dark Wizard hunter.
Theseus stays silent, but Newt knows by his face that he isn't buying any of it. Newt doesn't dare ask, but he knows that the scars of Leta's death still run deep into Theseus, which is why Theseus is desperate to defeat Grindelwald. He saw that Theseus failed to keep his composure in Berlin - it wasn't entirely Theseus' fault, Newt acknowledges, but the war has weakened Theseus' moral standings, and his emotions take over. Newt also knows his brother is not just doing this to avenge Leta. He didn't become an Auror, or fight a Muggle war, for his own emotional desires. It's also been for others.
But Newt knew that confrontation, especially with Theseus' anger brimming on edge, wouldn't lead Theseus and Dumbledore anywhere. Theseus, as a Hufflepuff, generally hated conflicts he knew would lead to fights. As much as Newt knew how much his brother hated his boss, Travers, Theseus would never indeed defy him because, morally, he knew Travers made fair decisions. He only did it to Newt or if he had to during work. Dumbledore, on the other hand, was a Gryffindor. If Newt was honest, he wasn't sure how Dumbledore would react. But both Newt and Theseus had respect for him as both a powerful wizard and their former professor, so to have a meeting to disagree with him felt awfully wrong.
"We don't know what Dumbledore's doing right now, Theseus. Let's have some faith in him - he's the only one who can stop Grindelwald, after all." Newt tries to relax the tension in the air. Theseus seems to have agreed, although Newt can't tell if he agrees or if he's too tired to continue. Head Auror things, I suppose…