At this moment Arcade is not able to access related data, it can only access the attributes of a single feature. In the future this (and more) will be included. Perhaps by using the ArcGIS API for Python you could update an attribute based on the statistics of the related table, but this would be a static result, while Arcade would yield a dynamic result. In other words, when the related data changes you would have to run the script again to reflect the changes in the map.
Thank you for your quick answer.
Just to make sure: you're saying it's not possible to show a feature layer's symbology based on a related data statistics despite its pop-up does?
The pop-up will provide access to the related data, but Arcade at this point (1.3.0 December 2017) does not have access to this related data. I can't be used for the symbology and nor for adding additional information in the pop-up (using Arcade).