怎样讲解英语中把句子变成复数形式,如This is her sister.其中her要变成their吗

怎样讲解英语中把句子变成复数形式,如This is her sister.其中her要变成their吗 请问That is her brother.变复数句,要不要把her变成their啊? 将下列句子变成复数形式：This is my sister. She is her aunt.变成复数形式 this is not my umbrella.把句子变成复数形式 把下列句子变成复数形式.This is a watch. 把下列句子变成复数形式.This is my pencil. 把下列句子变成复数句1、this is a bike 2.thatis her book3.that is an era 变成复数意义的句子、1.it is my dog 、2.she is her cousin 、3.is this her Emma is her sister变成同义句 把下列句子变成复数形式.It is your dictionary. 把下列句子变成复数形式.Is he a student?