Hill did not show any curiosity about Lynn's brother.
Lis occasionally mentioned a few things, all proving the child's extraordinary nature: he has now become the student of the only Level 40 Archmage in the Star Mountain Range, with a level reportedly already at Level 20.
No matter how outstanding Lynn's brother is, Hill would never inquire about another Archmage's situation; mages always maintain a certain distance and secrecy.
Actually, it's all about not wanting peers to know what one's trump card really is; if a mage can predict the enemy's actions in advance during a battle, there's practically no need to consider what failure is.
Just like Hill has been on the Toril Continent for so long, most people only think he's mysterious. After all, when Hill makes a move, he mostly uses the power of Divine Artifacts. The Wind Magic and Radiant Orb that he frequently uses are definitely not his trump cards.