First, have your parents keep the door to their room closed.
Their bed linen needs to be cleaned with Nature's Miracle (or similar product), then dry cleaned. There is always a lingering odor that we can't smell that will attract them.
How many litter boxes do you have and where are they located? (Thinking that one of the others may be keeping her from the box)
Can you keep her in your room whn you are away or are asleep, with a litter box? This may be the best temporary solution.
We usually keep the door to our bedroom closed when we aren't home. My husband accidentally left Zoey in the room once when he left for work (he leaves after I do). She let him keep that she was mad at him by peeing on his side of the bed. We did the proper cleaniing and make sure she is out of our room before we leave - and no repeats.
Skin kids - Jason, Kevin, Allison
Canine kids - Bennett & Bailey (always in our hearts), Riley, Banker, Rogue and Boz
Feline kids - Zoey, Talley and Abra