I would just do a simple reverse relationship.
Get the Maximum Study date for studies which meet the criteria. Call it [Date of most recent Engineering Conditions Study]
Lookup that down to the child records.
Then a new summary field of the Maximum Record ID# subject to the filter that the Date of the study equals the [Date of most recent Engineering Conditions Study] and any other filters like Study type = engineering conditions
Call this field [Record ID# of Most recently Engineering Conditions Study]
Then do a new reverse relationship where 1 document has many sites. On the right side use that field you just created. Then look up any fields you need from that Child to the Parent on that reverse relationship.
