While it is possible that the younger son is also the youngest son, it is not the emphasis of this sentence. If the writer had wanted to say the word "youngest" he would have added another word to show that he was the "most younger". IMHO, the sentence stands on its own with or without an expanded context.
It's possibly that it was marked incorrect because you used "the". The two words are not being used in a sentence and only a translation is asked for. It appears it would be different, and correct, if "the" was needed in an english sentence for it to be grammatically correct.