Skip..it sounds as though your sister had an anaphylactic reaction to either food, drug, or venom, (insect bite).
Whilst they test for common foods, they dont and cant test for everything.
It's possible to suddenly develop a food, drug,or venom allergy as an adult.
A friend of mine developed a penicillin allergy in his late 30's.
She just needs to keep her epipen with her always, (I should also,i'm allergic to bees) and either it will never happen again *fingers crossed*, or sooner or later she'll figure out what it is that's causing her anaphylaxis.
It might be worthwhile getting a referral to an Immunologist or allergist, if she hasn't already been referred by the hospital registra.
I doubt she has, because it's only been 10 days or so and they can't test with the antihistamines still in her system for days, she needs to see her GP to get a referral for more extensive testing.
She would have just had the basic skin-prick or blood tests in hospital.
It could have been some sort of uncommon insect that bit her, Skip.
I have seen an adult have an anaphylactic reaction to a green ant.

Also, theres exercise induced anaphylaxis, though this is quite rare.