seriously it never said the gender, it is just a sibling and the answer is brother.
동생이 is gender-neutral, so "younger/little sister," "younger/little brother," and "younger/little sibling" should all be acceptable answers.
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.
The correct term is "younger sibling". This is what I put in and it was correct.