What is the cause of the shortness of breath, fear of the deceased, and distress at the presence of siblings that afflicts my sister at dusk and midnight, even though all medical examinations are normal?

The Shari'ah has made it obligatory to seek treatment when an illness strikes. If the illness is not physical, the patient should see a psychiatrist. If that does not help, then it might be from the whispers of Satan, or possession by jinn, or magic. In such cases, Quranic verses that nullify these effects should be recited over the patient. This should be coupled with the importance of maintaining God's obligatory duties, the prescribed supplications (adhkar), and abstaining from unlawful acts.