The relationship between Andrews and Pirrie is well-known; take a look at Andrews' biography on this site, among other things. What makes you say it has "never been mentioned"?
William James Pirrie (later Viscount Pirrie of Belfast) was born on 31 May 1847, at Québec. He died at sea aboard Pacific Steam Navigation Company's Ebro, en route from South America to New York, on 7 June 1924 at the age of 77. (Sources: The New York Times, 9 June 1924; The Times (London), 9 June 1924; Moss and Hume's Shipbuilders to the World.)
What I meant by 'has never been mentioned' was that no one makes much of an issue that Lord Pirrie, one of the ships founders, and Mr. Andrews, one of the ships builders, were related. I just think its odd the two of the Titanic's most important founders were related. I'm assuming that Lord Pirrie checked over the plans for the ship as did many others but the relationship in family and the relationship in job is not acknowledged greatly even when working with the same ships, in this case being the Olympic, the Titanic, and later the Britannic (though without Andrews).
In fact, H & W had long been run by people who had family connections, going right back to the earliest founders, Edward Harland and Gustav Wolff. Both were related to a wealthy German businessman called Gustav Schwabe, whose wealth helped H & W expand.
One of the most important designers of Titanic was Alexander Carlisle, who was Lady Margaret Pirrie's brother. The odd man out was Edward Wilding, who had no family connection with Pirrie and learned naval architecture outside H & W.
Even at the lower levels, entry to the workforce was much easier if you had a father or another relative working there. H & W was a shining example of 'it's not what you know, it's who you know'.