How is it possible that my sister got pregnanted by his hiv/aids husband and she and the baby tested negative? Is it that the sperm does not carry the hiv virus?? please give me a good expalnation on this.

HIV is mostly carried in other cells of the seminal fluid, not the sperm itself. The transmission of HIV is dependent on many factors and it does not happen every time that an infected person has unprotected sex with and uninfected person. However, it always "possible" when there is unprotected intercourse within a mixed status couple. A baby can only be born HIV+ if the mother is HIV+ herself. Your sister has thus far been lucky but cannot count on that to continue if unprotected intercourse continues with her HIV+ husband.