One approach could be to put some attributes in your Customer_Orders entity to hold these values. You could then add some Before Commit and Before Delete microflow actions on the Fault_List entity to retrieve the Customer_Orders entity and then retrieve all Fault_List entities associated with that. You can then count up the priority of each and commit them in Customer_Orders.
I would use the custom string widget: https://appstore.home.mendix.com/link/app/1426/Mendix/CustomString
Then you can use a microflow as the source of the result. Depends a bit on your page build. You do not want this microflow fire for hundreds of lines. Then Robert sollution is better.