Many of the suggestions here are possible, but only in narrow situations (like technical writing). For example, I read above that "Younger brother and older sister" are now accepted. But (as a native speaker of English), I can't imagine a sentence in which this version would sound natural (except as a laconic reply to a question like "Are they the older brother and sister?").
For the most part, the answer to the question "Can I just leave out the articles: it sounds OK to me" is "No, you can't. English has articles, whether you find them difficult to use or not and they are rarely optional".