I am the younger of two. My sister is four years older than me.
[English Lesson]When you are comparing items in a 2-item list, you use the extension -er, not -est. -est is to be used where the list has more than two items.
Therefore, if you have, as I do, only one sibling, who is older than you, then you are "the younger of the two", and the sibling is "the older of the two". If you have more than one sibling, and you were the last to be born, then you are "the youngest of three".[/English Lesson]