How To Lookup fields from Child to Parent under certain condition

I know this has been answer many times but not finding the exact post/article to get this working.

Parent Record = Opportunity
Child Record = System

System can have multiple "stages" or even multiple system entries. These change over course of design.

Only want to display the system fields on Parent when system (child) has status of "current"
And not a related list but a group of fields that are combination numbers/text etc. 

Basically a reverse lookup but not sure best way to do this - Joined/Third Table?

Current Relation ship is -

Parent _                                                         Child  (Key Field here is Record ID)
Opportunity# (numeric key Field)                 Related Opportunity  (Reference)
                                                                     Rel-Opportunity Name (reference proxy)


Thanks for your help!