one brother is 哥哥，two brothers are 两个哥哥。 The nouns in Chinese do not change its form, be it singular or plural. But if you mean whether there is a Chinese word for sibling, then I am afraid there is no such a word. You have to say 兄弟姐妹。

no, you will always indicate the age, like "elder brother"... there's no just "brother"

brother: 兄，弟，哥 sister:姐，妹，姊 both but in a word: 我不知道一个字怎么说哎……手足？

In Chinese ，we usually use （们）to indicate the plural personal pronoun Just like 我…我们，你…你们 姐姐…姐姐们， 弟弟…弟弟们…