一、二楼的回答都很正确.比较级前面不能用twice来表示程度.如果说She is three years older than my 18-year-old sister就可以了.
twice,three times等只能放在as...as前面,或the age/size/length/strength/width/height...of前面.如：
I'm twice the height of you.我是你两倍高.
I'm three times as old as you.我的年纪是你的三倍.