Arthur knew Eugene had been plagued with jealousy over his dad. Whether it was with the twins or Robert, while the nature was different, the core was still the same: the fear of losing his dad to someone else. His dad's love, his dad's attention, the percentage of his existence in his dad's mind...
Naturally, Arthur understood this. No matter what, he still couldn't accept the thought of Hannah having a sweetheart—even if it was the son of the alpha he thought of as a role model and a brother. It might be a different brand of jealousy and possessiveness, but...it was close enough.
But Eugene was also an adult, so Arthur wasn't too worried. After all, despite his grumbling, Eugene never really restricted his dad, just like how Arthur never prevented Hannah from playing with Tobias.
That was why, Arthur was quite surprised at Eugene's reaction to the thought of Robert proposing to his dad.
