It would be great, if Graph delivers the owners of a SharePoint Site
GET https://graph.microsoft.com/v1.0/sites
or when asking for more details of a site
GET https://graph.microsoft.com/v1.0/sites/{site-id}
or with an extra method /owners, as with /groups/<groupid>/owners...
See more about that here.
https://sharepoint.stackexchange.com/questions/253282/how-to-get-site-owner-via-graph
Thx, Toni