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()}");
                }
            }
        }
    });
}