Important HIV question?

My friend's parents died of HIV and they were not aware that they had it until it turned into AIDS and killed them. My friend and her sis were born after the parents had HIV. My friend and her sis tested negative for any HIV but is it possible that the virus is undetected now but could show up in later years?

No, negative HIV antibody tests more than three to six months after possible infection are quite reliable, and HIV won't show up years later.

The only exception to this is tests done on babies less than 18 months old - these are a bit trickier to get reliable results from.

I dont know if this will help but I read just today that usually when people are exposed and become infected (80% or more) people get positive tests within 3-4 weeks, and it approaches 99% by 6 weeks.
If your friends were born to HIV positive parents and are negative I think they will stay that way. I remember when I was younger they Dr's said they couldn't tell if it was going to 'turn' into a positive for 7 years as it had an unusual way of laying dormant for this time period. With modern testing of HIV/AIDS Dr's have a more concise way of knowing how it goes from HIV to AIDS and the anti-bodies and markers our bodies produce when infected.
If one parent, just say the mother, is infected then the baby has a 30% chance of being HIV positive too this usually happens during the gestation/delivery or breast-feeding (I have never read anything that indicates an increase in infection to the child if both parents have HIV).; Im gathering your friend and his sister are not children- if they are negative now then it looks like they will stay that way, just educate them in ways to stay negative in the future. Check out a local Sexual Health Centre, they are free and have more knowledge than me ;)
Good Luck

Well, if your friends were more than six months old when they got tested for it, then they are not positive. It is very common for HIV positive people to have negative kids, even without the help of antivirals