"A brother of Paul McCartney" was probably Mike McGear, which would mean the group was probably The Scaffold - usually a trio rather than a duo. McGear said he changed his name because he didn't want to trade on being a Beatle's brother. Their big UK hit was Lily The Pink.

The other two members were - I think - John Gorman and Roger McGough. McGough has gone on to be a national treasure as a poet. I don't know what happened to the others.