Rather than select a relation, can't you just delete it using SketchRelationManager.DeleteRelation ?
First I display a list of all the relations in an active sketch. Then I want to be able to select the ones from the list then I can use "SketchRelationManager.DeleteRelation"
But first I need to identify them by their full name.
If I select a sketch line, Solidworks displays a list of it's relations in a list box on the property manager as: Horizontal1, Tangent2, etc...
The GetRelationType method only exposes the "type" not the full name. However the real question is can I select a relation by using "ModelDocExtension.SelectbyID2"?
If I record a macro then edit it I see this:
Part.Extension.SelectByID2("Unknown", "MANIPULATOR", 4.44499999999806E-04, 8.77048391607468E-02, -3.79004434588306E-03, False, 0, Nothing, 0)
not too helpful...