I Googled the symptoms. They are as follows:
Behavioural: compulsive behaviour, agitation, compulsive hoarding, hypervigilance, impulsivity, meaningless repetition of own words, repetitive movements, ritualistic behaviour, social isolation, or persistent repetition of words or actions
Mood: anxiety, apprehension, guilt, or panic attack
Psychological: depression or fear
Also common: food aversion, nightmares, or repeatedly going over thoughts
I think she is just being wierd. I myself sometimes get scared by looking in the sky. I think that what if I leave the Earth and wander in the space and I don't have any oxygen to breathe. Ask your sister what makes her scared of toilet paper. Don't make fun of her or she will not share her thoughts with you.