I'm worried that my sister might be pregnant?

She had sex with a guy about 2 months ago and hasn't gotten her period since. She was off her birth control when she had sex, she said they used a condom but she seemed doubtful of the protection. She took 2 home pregnancy tests and they were both negative. She hasn't had any morning sickness but I have noticed that her appetite increased and she has been eating alot more. Is there any possibility that the tests were wrong and her symptoms are delayed?