Is this factory----your mother once worked.

如果这个地方factory 前面有一个定冠词the,则可以用in which
这个地方如果用陈述语序,则应该是：this factory is where you mother once worked.where引导的是表语从句,不能用 in which替换,而加上the以后,则结构发生变化,this is the factory where your mother once worked. this做主语,the factory作为表语,where引导的从句作为关系副词引导的定语从句,此时就可以用in which 替换where了.
需要提醒注意的是,只有在定语从句中, 才能用in which或者at which 替换where这个引导词.
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