They're not talking about anxiety that makes sense here.
You being scared of your sister dying makes sense.
They're talking about irrational anxiety, like having a panic attack, because your stomach feels slightly funny, or because you slept weird, etc.
Those irrational anxieties are often times hard to pinpoint to an actual cause.
Like your sitting at home on a sunday and suddenly have a panic attack, without any immediate cause or threat.
It applies to both. Being scared of one's sister dying has a definite cause you can pinpoint to, but that still doesn't explain the racing heart, or stomach sickness, or anything else. There's no "tiger in the bushes" reason why your body needs to dump a whole bunch of adrenaline in you right now.
Learning that you can calm your body down separately from wanting to think about the actual issue at hand is a good lesson.
