To create a new relationship type, use RAMSession.createRelationshipType(String relName, String reverseName). Only a repository administrator can create relationship types.
//Create a new relationship type along with it's reverse type
RAMRelationshipType newRelationshipType = session.createRelationshipType("New Relationship Type", "New Reverse Relationship Type");
session.put(newRelationshipType, new NullProgressMonitor());