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.

For example, with an AA F LHR-JFK tix leaving from LHR T3, I was able to access the CX F lounge in LHR T3. You don't need status for this.