What it sounds like your sister had has a name... APS or Antiphospholipid Syndrome. It is sometimes called Anticardiolipin Antibodies (ACA) or Hughes' Disease. Basically, with people who had APS, the blood turns "sticky" (you may hear it called sticky blood). With APS, the platelets stick together and clot. The clot can burst, it can go to the heart (causing a heart attack) or it can go to the brain (causing a stroke). If the clot bursts and the platelets are counted, it may look like the patient has low platelets and it is therefore easy to misdiagnose as Idiopathic Thrombocytopenia Purpura (ITP) or low platelets of an unknown origin.
I know this because I was misdiagnosed for over 8 years with ITP when further tests showed that I had APS.
The reason that the doctors asked you to check into this for the rest of your family and be tested is because APS is an autoimmune disease and they can be hereditary. In addition, you may find autoimmune diseases that vary within the family. For instance, I had APS while my younger sister has Krohn's Disease (effects the bowels, etc...). Autoimmune diseases also, generally, effect more women then men.
Please visit http://www.aarda.org/ this is the American Autoimmune Related Disease Association, Inc. They can provide you with information on all of the various Autoimmune Diseases and you can also request further information. As for your neice, make sure she is aware of her abnormal tests and even though she has since tested normal, to have yearly tests (or even every 6 months). Autoimmune diseases can mask themselves as other things and if a doctor is not interested in testing for autoimmune diseases (and it runs in your family) I would seek out another doctor.
As I said, I was misdiagnosed for over 8 years and had I stopped with each one of the doctors who told me it was nothing, all in my head, or that it was ITP then I would still be misdiagnosed!
Good Luck, and I am sorry about your sister.
I am sorry to hear about your sister. These antibodies can sometimes cause heart attacks, stroke, and blood clots in other places as well. The usual remedy is to take blood-thinning medications life-long. The tests measure the levels of antibodies and this value can vary from time to time and from lab to lab. Unless the values are very elevated, there is usually no need to do anything. So, if your niece's value was within the normal range on repeat testing, this is quite reassuring.