Where is the woman who is Li Lei's mother?
Under the tree是地点状语,对地点提问就要使用疑问词Where.
显然此句被提问对象是那个woman,但为了保持原句的表达内容不缺少,加上定语从句who is Li Lei's mother来修饰主语woman
这句话翻译为：那个是李磊母亲的女士在哪里?