When he was young, his brother was posted to Sydney, so the whole family moved there.

was 是 be 动词的过去式，在这里构成被动语态。 posted 是动词 post 的过去分词，post 本意是"邮寄、张贴"，引申为"派遣、调任"。 to 是介词，表示方向或目的地，在这里引出被派往的地点。 was posted to 是被动语态结构，表示"被派往/被调任到"某个地方。