A DoD Dependent ID card should suffice. Unless you are claiming that since your ex-wife's third cousin twice-removed is in an ROTC program that you should qualify as a "family member", if so speak to the customer service rep at HD to see if you qualify.
The serviceman in question is my stepson, who just shipped out aboard the Princeton with the Nimitz battle group, so my wife and I aren't dependents, if that's the only family members they're targeting.