set the recordsource of the subform to include all the rows in the relationship except the record displayed on the main form. I'm not really sure what the logic defining "the matching relationship" looks like, but I've taken a guess below: (this is all one query but I've broken it up a bit with comments for understanding purposes - remove the comments to use it)
'get all persons in a given relationship
SELECT p.Person_ID, First, Last
FROM Person p INNER JOIN Relationship r ON p.Person_ID = r.Person_ID
'define given relationship
WHERE Relationship_ID = (
SELECT Relationship_ID FROM Relationship
WHERE Person_ID = Forms(nameOfYourMainForm)!nameOfTextboxInMainFormContainingPersonID)
'and the person isnt the person displayed on the main form:
AND p.Person_ID <> Forms(nameOfYourMainForm)!nameOfTextboxInMainFormContainingPersonID;