QGIS Python Script for loading MS-Server Tables and create relates and Attribute Form Widgets

Question

I have the following Python script for QGIS3, which is supposed to load spatial and non-spatial tables into a new project. Relationships are then to be set and certain links are to be created in the attribute form. Loading the tables works so far, but I cannot access the relationships (as much as if I drag the tables into QGIS by hand. By relationships I mean what is under: Project/Properties/Relations/AddRealation Here the tables are available when added manually, but unfortunately not after running the script.

Why is that?

SCRIPT:

from qgis.core import (
    QgsProject,
    QgsVectorLayer,
    QgsRasterLayer,
    QgsVectorLayerJoinInfo,
    QgsRelation
)

#DATAPATHES
uri = "dbname='XXX' host=XXX estimatedmetadata=true srid=4647 type=Point disableInvalidGeometryHandling='0'"

#OSM
osm_layer = QgsRasterLayer(r"type=xyz&url=https://tile.openstreetmap.org/{z}/{x}/{y}.png", "OpenStreetMap", "wms")

#DGM
dgm_layer = QgsRasterLayer(r"\\XXX\DGM1_NI_Hoehendaten.tif", "DGM1", "gdal")

#CREATING GROUPS
root = QgsProject.instance().layerTreeRoot()
basegroup = root.insertGroup(0, 'BaseMap')
zuogroup  = root.insertGroup(0, 'ZuordnungsTabellen')
refgroup  = root.insertGroup(0, 'ReferenzTabellen')
maingroup = root.insertGroup(0, 'HauptTabellen')

#MAINTABLE
IEK_EREIGNIS    = QgsVectorLayer(uri_ereignis + " table=\"dbo.IEK_EREIGNIS\" (eGeom)", "IEK_EREIGNIS", "mssql")
maingroup.addLayer(IEK_EREIGNIS)

#REFERENCETABLES
IEK_AKTIVITAETANZEICHEN = QgsVectorLayer(uri + "table=\"dbo.IEK_AKTIVITAETANZEICHEN\" ", "IEK_AKTIVITAETANZEICHEN", "mssql")

IEK_AKTIVITAETSART = QgsVectorLayer(uri + "table=\"dbo.IEK_AKTIVITAETSART\" ", "IEK_AKTIVITAETSART", "mssql")

IEK_ZUO_AKTIVITAETANZEICHEN_EREIGNIS = QgsVectorLayer(uri + "table=\"dbo.IEK_ZUO_AKTIVITAETANZEICHEN_EREIGNIS\" ", "IEK_ZUO_AKTIVITAETANZEICHEN_EREIGNIS", "mssql")


#TO GROUPS
#ReferenzTabellen
refgroup.addLayer(IEK_AKTIVITAETSART)
refgroup.addLayer(IEK_AKTIVITAETANZEICHEN)
zuogroup.addLayer(IEK_ZUO_AKTIVITAETANZEICHEN_EREIGNIS)
#BaseMaps
basegroup.addLayer(dgm_layer)
basegroup.addLayer(osm_layer)

#Zoom DGM1-Layer
iface.mapCanvas().setExtent(dgm_layer.extent())
iface.mapCanvas().refresh()

Now the Layers are loaded and visible, but not accessible in Project/Properties/Relations...