why does it take Three to say oldest/youngest?
I thought two was enough to be able to have a oldest of the two?
You've got two younger brothers, the youngest one is younger than the other.
Is that not correct English?
I've got two brothers younger than I am. The older of the two is ABC and the younger one is XYZ. They are both pains in the butt.
Comparative (-er) involves only two being compared.
Superlative (-est) involves more than two in the comparison.