And there,almost---in the big chair,sat her little brother,w

,almostXXXin the big chair,作为插入语不看.
句子是there sat her little brother,是完全倒装,和there be 句型是一个道理,顺过来就是her little brother sat there,主语brother,谓语 sat.
后面,who跟的是定语从句修饰brother的,值得注意这里倒装的目的就是为了让brother后面跟的是定语从句,如果不到装这个定语从句就不知道如何放了(因为有插入语)