public IReadOnlyList<Definition> GetRelatedDefinitions( 
   Definition definition,
   DefinitionRelationshipType relationshipType
)

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

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