Hello
Here is a sample scripted GP tool that handles selections between objects sharing keys, it has no dependency on relates:
http://www.arcgis.com/home/item.html?id=e638afe0695a4ad38388cb8d9b350446
Regards
Bruce, this is a great tool. I was attempting to modify a bit so that selections could be created in the dialog via Query Builder and passed to the tool prior to the main execution, but running into some issues. SQL Expressions are not being honored, and all records in the data set are selecting. Any tips?