Also, it depends on what you want to say. 兄弟 means both brothers and sisters like catherine said, but if you want to be more specific you could say that you've got an older brother/s and a little sister/s by saying "兄と妹がいます". You could also say "弟と姉がいます". These are usually used after you've said "兄弟がいます" and the other person is wondering what kind of siblings, because 兄弟 is pretty vague.

有点小问题。首先，“兄（弟）と姉（妹）”日本人一般不会这样用，一般来说都会用“私には兄弟がいます”，因为在日本“兄弟”这个词就指代兄弟姐妹。另外就是あります用于不具有意志的东西。人的话是有意志的所以要用います。