兄弟 includes you. So if you have a sister, 兄弟が2人（ふたり）います。 When you're talking about your own sister to someone else, it's usual to use 姉（あね） rather than お姉さん. So you'd say 姉が一人います。 You need が because it marks 姉 or 兄弟 as the subject of the sentences, which literally translate to "there is one older sister" or "there are two siblings" I don't know whether the reversed word order is okay, though. Maybe a native speaker can help with that part of your question. I hope that helps you!