Entity relationship conundrum

I've gotten myself all tangled up in the relationships of my entities and am now stuck:

1. Case to Contact is N:N.  A Case has many Contacts who are "family members" on the Case.  A Contact can be associated to many Cases (often the case in social services).
2. Case to Purchase Order is 1:N.

I need to create a Purchase Order and select one of the family members.  Do I have to create another entity to hold the join between the Contacts related to the Case and the Purchase Order, or is there a way to do this that I'm totally missing?