In order to publish a feature class, with related records from ArcGIS Pro (i'm using 2.0), one needs to do so via the ribbon 'Share' menu and NOT by right clicking the individual layer via the Contents pane.
At the moment, the differences between these methods are not clear in the documentation:
Web feature layer—ArcGIS Pro | ArcGIS Desktop
... Nor is there any mention to the implications for related tables.