@AncientWatcher
You seem to have misunderstood why he decided for these test.
He assumed that, because of the granddaddy being super traditional, that the girl would've been taught how to do housework properly, which includes cooking.
The blond looks and acts the most like a boy. But this could be what the author wants you to believe, so I think it's either that the blond is the girl or both are.
I do not think anyone outside of the main 3 of them has ever said that there is a boy (and the MC brings up the idea first ) I bet that it will end up that one of them was just a tomboy
I'm guessing that their family has a weird rule that if there are twins the younger one has to crossdress. And somewhere down the line the older one decide to do so as well for plot reasons.