Not owning up... and how to address the upset

[Goddit]

Run it by another family member on her side. the story might get back to her indirectly and you may not have to do anything. If the worst comes to the worse, consider this approach....
Start by saying that you so appreciate you relationship (friendship, love, whatever) and that you would hate for anything to threaten that but something has been niggling you for a while and could you explain. Hopefully the answer will be 'yes', at which point you present what you see as your side of the story (use those terms, for how you see it may not be how she sees it). If she values your friendship (your opening offer), she will fess up (assuming she has been wayward); if she doesn't then you may need to cut the tie and count your losses.