well there is one workaround I can think of... but it isn't pretty.
Make a new template and a new forum. associate that template with only that forum. add a metarefresh to the header of the forum template and have it take you to the page you want.
Make a new template called "Sister site"
In the template, go into the header insert and add
<META HTTP-EQUIV=Refresh CONTENT="1; URL=http://www.******.com/">
Make a new forum called whatever the name of the sister site is.
Go into the forums settings.
Associate that template with that forum AND add
If not automatically forwarded, click <a href="http://www.******.com/">here</a>.
Click update and go into your forum summary, there should be the forum you made. Click the link and it should forward you to the site you wanted after a little loading.
An example of this can be seen at
http://www.alforadmin.com/forums .
Click the forum called "UBBDEV"
I'm gonna look into an htaccess version of this in a second