在这理应是后面的那个.
her sister's and Kate's
顺便说一下,
也有her sister and Kate's 说法,不过不能用在skirt上.因为两个是不可能共一条skirt的.
请对比下面两组,会清楚些:
her sister's and Kate's room are ...
her sister and Kate's room is ...,
room都是单数,但动词的和数不同.前者是两个各的,后者是共一个的.