QP.
The best way of looking at this (not at all complex) issue is that there's 2 lounges OW members can access, a Sapphire (blue symbol) and Emerald (green symbol) lounge. Usually, the Emerald lounge is designated as a First class lounge, but there's no domestic first class on Qantas, only business.
Thus, if your family member has a card with a blue symbol and the QP sign also has a blue symbol (they usually do), then you've matched up your access rights
As an addendum, the "Business lounge" has a sign with a green symbol and that matches the Diamond MPC card.
The Business lounge is in name only, they could call it a First class lounge, but I think that'd probably confuse paying business class passengers
