Like others have alluded to, the angle your sister is coming from only makes any sense because of how different related conditions are referred to, because of specific quirks in how and when each part was figured out.

It's kind of like saying 'diabetic ketoacidosis is a symptom of diabetes, so there should be other conditions which cause it.' Except, 'diabetic' is in the name, while 'HIV' isn't in the name that stuck for the condition it can cause, AIDS.