How to tell Google When Multiple domains are Related?

If you have two similar websites like ebay.com and ebay.co.uk and they are linking each other then, will Google view this linking as paid or unnatural? Matt Cutts has given the answer of about ccTLDs and sister websites here http://www.digitaltalks.info/how-to-tell-google-when-multiple-domains-are-related-matt-cutts/