Looks like you need these relationships:
Owners::__pkContactID = Sites::_fkOwnerID
ResponsibleParties::__pkContactID = Sites::_fkRPID
While this looks like you need two tables, one for owners and one for ResponsibleParties, you do not actually need two tables, just two Tutorial: What are Table Occurrences?.
In Manage | Database | relationships, make a new table occurrence of Owners by clicking it and then clicking the duplicate button (2 green plus signs). You can double click the new occurrence box to get a dialog to appear where you can rename the new occurrence box to be ResponsibleParties.
We have not duplicated a table. Instead, this is a new reference to the same table already present in your database.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
You guys are quick! That was exactly the right answer. Thanks.