（1）重读are
Mary and Jane “are” sisters.强调Mary和Jane的的确确是姐妹,相当于Mary and Jane are really sisters
（2）不重读are
“Mary and Jane” are sisters.强调是Mary和Jane是姐妹,而不是别人是姐妹
Mary and Jane are “sisters”.
强调Mary和Jane是姐妹关系,而不是别的关系