public enum DefinitionRelationshipType : System.Enum, System.IComparable, System.IConvertible, System.IFormattable  

public async Task ObtainingRelatedDefinitionsFromGeodatabase()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
    {
      // Remember the qualification of DatabaseName. for the RelationshipClass.

      RelationshipClassDefinition enterpriseDefinition = 
      geodatabase.GetDefinition<RelationshipClassDefinition>("LocalGovernment.GDB.AddressPointHasSiteAddresses");
      IReadOnlyList<Definition> enterpriseDefinitions = 
      geodatabase.GetRelatedDefinitions(enterpriseDefinition, DefinitionRelationshipType.DatasetsRelatedThrough);
      FeatureClassDefinition enterpriseAddressPointDefinition = 
      enterpriseDefinitions.First(
              defn => defn.GetName().Equals("LocalGovernment.GDB.AddressPoint")) as FeatureClassDefinition;

      FeatureDatasetDefinition featureDatasetDefinition = 
        geodatabase.GetDefinition<FeatureDatasetDefinition>("LocalGovernment.GDB.Address");
      IReadOnlyList<Definition> datasetsInAddressDataset = 
        geodatabase.GetRelatedDefinitions(featureDatasetDefinition, DefinitionRelationshipType.DatasetInFeatureDataset);
      FeatureClassDefinition addressPointInAddressDataset = 
        datasetsInAddressDataset.First(
              defn => defn.GetName().Equals("LocalGovernment.GDB.AddressPoint")) as FeatureClassDefinition;

      RelationshipClassDefinition addressPointHasSiteAddressInAddressDataset = datasetsInAddressDataset.First(
              defn => defn.GetName().Equals("LocalGovernment.GDB.AddressPointHasSiteAddresses")) as RelationshipClassDefinition;
    }
  });
}

public async Task GetFeatureClassesInRelationshipClassAsync()
{
  await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {
    using (Geodatabase geodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri(@"C:\Data\LocalGovernment.gdb"))))
    {
      IReadOnlyList<RelationshipClassDefinition> relationshipClassDefinitions = 
        geodatabase.GetDefinitions<RelationshipClassDefinition>();

      foreach (var relationshipClassDefinition in relationshipClassDefinitions)
      {
        IReadOnlyList<Definition> definitions = 
          geodatabase.GetRelatedDefinitions(relationshipClassDefinition,
            DefinitionRelationshipType.DatasetsRelatedThrough);

        foreach (var definition in definitions)
        {
          System.Diagnostics.Debug.WriteLine(
            $"Feature class in the RelationshipClass is:{definition.GetName()}");
        }
      }
    }
  });
}