I will start off by saying that HIV is not an autoimmune disorder
. HIV is a virus that infects a specific type of white blood cell called the T-cell. If left untreated, HIV can reduce the number of T-cells to a level that allows infections that would otherwise be fought off by the body's defenses to run unchecked. If this happens, then we say the person has an acquired immune deficiency syndrome (also known as AIDS). It sounds like your sister has contracted HIV, which is a lot more common of a disease than people think. Worldwide, HIV effects men and women similarly, in the United States it effects slightly more men than women.
On the other hand, autoimmune disorders are a group of conditions in which the body's immune system is over active (as opposed to under active as in AIDS) and it attacks the body's tissues. Depending on the disease in question, the immune system can do serious damage to the body's organs. Although it is not a steadfast rule, most autoimmune disorders effect women more than they effect men.
I suggest that you bring these types of questions about your health up with your primary care physician
. the two of you can discuss measures that can be taken to protect you from being exposed to HIV. Good luck.