“你妹妹午餐吃什么”用英语怎么说？

What does your young sister have for lunch?

What do your *** have for ***?

What does your sister have for lunch?
What does your XX have at (time)?

What did your sister eat for lunch?
你妹妹午餐吃什么,这是过去时，表示问已经吃过什么

你的某某什么时间吃什么，如果这顿饭还没有吃，或者表示一帮情况下吃什么，那就是
What do(does) your 某某 eat for 什么（例如早餐breakfast，午餐lunch晚餐dinner等）