My sister maybe pregnant??? Please help

So my sister told me that her period is late and usually were around the same time she told me that the week before her period was suppose to come she had had sex with her ex bf so when she was two weeks late I got her to take a pregnancy test and it came back negative then last week was 4 weeks and still no period so she took another test and it still came back negative this is the fifth week and still nothing the next step is taking her to the doctors and getting a blood test but can she be pregnant and it still not show cause she's had really bad mood swings just recently and like she's showing symptoms that she could be but i dont know please help