Customise Contacts Relationships Tab
You can achieve this through following ways:


Create ACL rule through UI to View Contact of specific group.
Create an extension and use pageRun hook to alter the $rows send to smarty template.
Create an extension and use aclWhereClause hook to restrict contacts to show.
Probably not sure apiWrappers or selectWhereClause might also be used.


Note: Using #1 and #3 will be affected on all pages/forms/reports.

HTH

Pradeep