I don't think it's a good idea to analogize Chinese from English, as 景洛弘 said. Actually, there is a word which can mean "sibling". It's 同胞(tóng bāo).同胞 could mean:
- people born of the same parents.
- people from the same race or nation(here, the mothercountry is used as an analogy of "parents")
But in general, we don't use this word with the 1st meaning, instead, 兄弟姐妹 is more common to see.
Why should we use one or two special characters to express "Sibling" in Chinese, as in English? It has its own advantages. In Chinese, you could say
我们是姐妹 We are sisters.
我们是兄弟 We are brothers.
However, you could say,
In this case, if you are male, the listener will know that you are the younger borther and the other is your elder sister.
We also have words like
骨肉 Literally means: bone and flesh. Actually means: parents and children
as in words
骨肉团圆 The parents reunite with their children.
In English, could you express these meanings with one word respectively? I mean,
我们是姐弟 We are [?]
骨肉团圆 [?] gets reunited.