The most accurate translation is "younger sibling," even though it may sound awkward in English, as we are accustomed to saying "brother" or "sister".
By the way, Korean kids refer to older friends and cousins as "brothers" or "sisters," so they tend to do the same in English. Korean English teachers probably do not notice or focus on this subtle aspect of Konglish.
Very good point. In fact, form a functional perspective, it more correct to leave 'younger' completely out - no native English speaking child would say 'younger sibling', they would more naturally say 'my/our brother/sister', though it doesn't translate literal that way.