Are pregnancy tests 100% accurate?

My sister got 2 negative HPTs and 1 negative blood test for pregnancy. Her period is 2 weeks late and the possible day of conception was 40 days ago. Blood test was taken at 38 days and Hpt were around 32. She has symptoms such as wetness down there, breast pain, cramps and no period. She's never had intercourse before, and she's still not convinced until she gets her period but she's super worried. The doctor said its stress, were just wondering because she's still super worried and ours parents would kill her...