You're right to let it slide off your back; your sister is doing what you expect--blame-shifting for all she's worth. Poor her.
When XAH does this kind of crap to me, I try to remember my mantra:
"La pluie de tes insultes n'atteint pas le parapluie de mon indifférence" (the storm of your insults cannot penetrate the umbrella of my indifference).