同学你好，该知识点来自沪江网校《英语零基础直达职场全能【年中特惠班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
elder表示“前辈的”、“年纪较长的”，仅用于同一家庭成员的比较；older则不限于此。例如：
My younger son is five and my elder son is nine。我的小儿子5岁，大儿子9岁。
Tom is two years older than I.汤姆比我年长两岁。

elder在句中只用作定语，不作表语, 而且不说elder than；older两者皆可。例如：
My elder sister does her homework all by herself. 我的姐姐独自做作业。
I think his mother is older. 我认为他的母亲年龄大。

elder只能用来指人；older既可修饰人，也可修饰物。例如：
My elder brother joined the army when he was only fifteen.我哥哥年仅15负就参了军。
This dog is older than any other dogs here.这条狗在这里是最老的一条。

所以要说，My sister is three years older than me.

祝同学学习进步~