“ First, {{plain sister}} links to {{TALKSPACE}}:{{PAGENAME}} which is different than always linking to the main talk space as this template links to Talk:{{PAGENAME}}.
Second, {{plain sister}}, does not implement a replacement for the |title= parameter of this template, so it cannot link to other main talk pages. It is commonly used to link to the talk page of a main page that is a base page of the current page (so things like chapter subpages can link to the edition documentation of a base page) . ”