And there, almost___in the chair,sat her little brother,who never had to be told to be quiet.

第6版的牛津高阶上有写：
be lost in sth（1038页）
全神贯注；沉浸于；to be giving all your attention to sth so that you do not notice what is happening around you.

我询问我的老师说可以这样翻译：
在那边坐着她的兄弟，几乎整个身子都陷在椅子里，他是个不用别人说就会保持安静的人。

我觉得这里lost有了比喻的含义（高阶字典里没一个解释对得上），这里解释为“沉默，保持安静（安静的感觉人间蒸发了）”

那里，安静地坐着我的弟弟，他从未被要求安静

almost___in the chair是插入语
who never had to be told to be quiet是定语

And there （almost___in the chair）sat her little brother,（who never had to be told to be quiet.）

新手提问怎么都不给分。。。