Those people are related. They're part of the powerful Bowmore family (which built Bowmore bridge) and are part of the alliance (with Rowan House and Clelland House) to take down Lord Strangford (p. 29).
All of the powerful noble families have members in positions of high power throughout the Imperium. You can safely assume that everyone with the same last name of a powerful House is related and all attend the family council meetings where they hatch horrible schemes. :)