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

Tom and linda are brother and sister aTom and linda are brother and sister and,mr brown is ( )(they)father. They are my parents,my sister,my brother and I. 改错-is your brother and sister workers?yes,they are Are they sister and brother?要求是肯定回答! 改错-Is your brother and sister workers? Yes,they are 英语翻译可否回答为They are brother and sister. They are his sister and brother.把his sister and brother提问 They are sister and brothersister and brother为什么不用复数? Tom and Linda are brother and sister,and Mr Brown is ___ (they) father. two sister, beautiful and bright, they always run, but never meet. what are they? they are ________ (twin) sister. My brother and sister are not in the same school.They are in ____ schools. Tom and Mary are brother and sister.they ____ in a large house. Tom and May are brother and sister .They always do homework t_____ tom and linda are brother and sister.they livea big house. they aren‘t brother and sister,（ ）填反意疑问句 为什么是are there 还是are they?they aren‘t brother and sister,（ ）填反意疑问句 为什么是are there 还是are they?为什么 急 Liu Ying and Liu Li are _____.They look nearly the same .Atwin sister B twins sisters C twin sister they are my sister and brother and they can help me study .这个句子通顺么?求改错。