Their conversation was clearly not done in the situation where they need to pretend (maybe there's noone nearby to overhear), because with that question the brother just pointed out the "strangeness" of her remark instead. He could have responded differently.
And you know there were no others because...
And you know the tone and look on his face when he said that because...
Also,how do you even know if they even were awake and getting ready for school at the same time?,because clearly the usual trope of ensuring they don't go together at the same time from the same direction is gonna come up.
For all we know,he had the biggest darn smug look on his face and said that to put her on the spot,which is a stretch further then saying he did as a clever way to remind her to not blurt things out.
But assuming off the bat that the conversation is unrealistic and therefore the author is stupid with no solid references to situation at hand?...really now?...