Sister Site is an invention of Wards Wiki. See Sister Sites.
Now we propose a user-defined concept, named Partner Wiki as a Community Solution, that could be supported by a Technology Solution (with realtime creation of Partner Pages and Partner Wikis)
The pseudo-icons <=>, <-> at the bottom of the page stand as examples for a proposed notion to make user-defined "Sister Sites", which we could call Partner Wikis, if you prefer this notion.
Partner Pages are bidirectionally connected pages, where their respective links are listed at the bottom of the involved pages.
Partner Wikis are Wikis connected by Partner Pages, which in the weak form have "<->" connections and in the strong form have '<=>' connections.
In the strong form "<=>", the coupled pages are bidirectionally linked namesakes as used in the classical Sister Site concept.
In the weak form '<->', the coupled pages are "only" visibly bidirectionally linked pages.
<=> www.usemod.com 
,.. This is an example of a user-defined Namesake Partner Page Link list
<-> Sister Sites,.. This is an example of a user-defined Partner Page Link list
See original on c2.com