你妹妹午餐吃什么用英语怎么说?

由于上面楼主没有说明问的是哪一时刻或状态 所以作一下两种翻译
what does your younger sister have for lunch usually ? (这是一般的时态，问的是经常性习惯性的动作或者状态)
what did your younger sister have for lunch ？（这问的是 过去的某一次的动作）

What did your sister have for lunch?

what did your sister have for lunch?

what does your sister usually have for lunch?