Parent/child each multiple choice value and corresponding line item

I have 3 lists:

1) "Branch"

2) "Survey" > looks up [Sites] (multiple choice) in "Branch"  

3) "Response" > looks up [Sites] (drop down) in "Branch", and [Survey Title] in "Survey"

A Survey item indicates which [Applicable Sites], from the multiple choice values, must respond by a [Due Date].   The Response list contains 1 Response/Survey. 

I need to periodically send reminders to the Sites which have not yet responded.

I created a workflow in the "Survey" list that splits the choice field and I was able to have a For Each Site action to work (Regular Expression to split and save in a Collection Operation), then ...  how would I compare each value in Survey, to return the missing sites items in Responses?

Thanks,

Monica