engbel and uH netherlands being an over protective brother there we go
Arthur knows that Stijn is protective of his little sister, so that's why they decide to keep their relationship hush hush at first -- not only to make sure that they had their privacy from various (very nosy) nations, but also from her brother and the possibility of him being very displeased with Lotte's choice of a man.
But eventually, someone has to catch them kissing (and nearly shagging, Arthur has to admit) in the broom cupboard at the meetings, and it's only a matter of time before Stijn finds out.
However, the man simply walks up to Arthur after the meeting (and though Arthur has always said that tall people don't intimidate him, seeing Stijn coming towards him and towering over him is a bit of a challenge not to try and run off from), and crosses his arms, looking him up and down.
"You know if you hurt her, I'll be after you."
"Ah....--yes."
(and despite how overprotective he is of his little sister, Stijn knows to respect his sister's choices...at least at first)
