Google Says Linking Brand Websites Together At Reasonable Scale Is Fine

"This seems pretty common, I don't see a problem with it when done at reasonable scale. My expectation would be that the site does better overall with a single presence, but splitting shouldn't cause problems. So yea, this is fairly common amongst larger brands. But he does think it may make sense to put all the sites together in some cases. I mean, that would depend on the site. If a news organization owns several news properties, those properties would probably need their own brand names and sites."

"Back in 2013, Matt Cutts formerly from Google, said be careful linking too many sites together. The question then was "If I have 20 domains, should I link them all together?" The short answer is, most likely no - you should not link them all together. Then in 2014, he said be careful linking sister sites together. But I guess the advice has changed since then..."

Linking sister brand websites via a partner or brand page is a common and acceptable practice when implemented at a reasonable scale. A single, unified web presence often performs better overall, but maintaining separate sites for distinct brands typically does not create problems. Combining sites can make sense in many situations, while certain industries or use cases, such as multiple news properties, may require independent brand sites. Historical guidance warned against linking many domains together, advising caution with extensive cross-domain linking, but current guidance is more permissive when done sensibly.

Read at Search Engine Roundtable