Yes, if you have a OW J tix for a flight that day (IB J in your sister's case), she can access any OW J lounge subject to local requirements, e.g. in LHR, you can only access a lounge in terminal from which you're departing.
So someone flying CX in J can access the Admirals Club at LHR T3?
(before you scoff & chase me off the board, i'm thinking about an upcoming family trip and although the CX lounge sounds very swanky, i'm sure my 2 yr old son would prefer to be looking out of a window.....)