I'm not a doctor, but I used to get hives and they would come and go so fast it was ridiculous. I would have them, go to see the doctor, and they'd be GONE! But finally, we found out that I was having an allergic reaction to something. But I couldn't tell you why your sister is getting them but maybe she's allergic to something.
They are usually an allergic reaction, but some get them where there skin is sensitive and being rubbed by clothing, others get hives from stress. en.wikipedia.org
They are an allergic reaction normally accompanied by a itchy feeling.
Hives is also known as urticaria is a skin disease it is red and sometimes itchy bumps on skin. They are generally caused by an allergic reaction to any food or medication. These days many treatments are available in the market to treat the condition of hives naturally and one of the trusted medication in the market out there is oxyhives which is a homeopathic blend of natural ingredients. Oxyhives cures hives naturally without any side effects...