"They are sister and brother"sister and brother为什么不用复数?

If there is one brother and one sister,
I would say: They are brother and sister.为什么sister and brother不用复数？会读吗？不会的话我翻译下吧If there is one brother and one sister,如果只有一个brother和一个sisterI would say: They are brother and sister. 我就会说： They are brother and sister.不会再问吧