Just looking at some of the other photos on that site, This is the car built by RED at the same time as the CG ford Sierra.

http://www.flickr.com/photos/1678887...n/photostream/

They wern't sister cars as such, but Dougie told me they worked together on the susspension designs and layouts of the two cars. Both were unique and not repeated as the sierra cosworth was released within a couple of years and group A rallying was established.
As far as I'm aware only the CG ford car of the two still exists.