Could My sister be pregnant??? Help!?

My 18 year old sister had sex on the 26th of last month with her boyfriend. (She had a pregnancy scare in March this year) She decided to use the same method that almost got her pregnant last time. The pull-out method! (Shes not very smart) So anyways, I want to study to be a gynecologist or really any kind of doctor when I get to collage so I'm doing all the research and things for her. (Signs and everything.) Last time, I bought her 5 pregnancy tests. She took three after her missed period and they came up negative. She said she wasn't ever having unprotected sex again! (Well she did) Her friend thought she was pregnant so my sister lent her a pregnancy test. She only has 1 left now. (Her friend was only 1 week after sex and her test was negative.) I have done a lot of research on pregnancy symptoms and signs, I have a feeling my sister won't be so lucky this time. She has plenty of symptoms. She is now 5 weeks in to the pregnancy (if she is pregnant) I keep trying to convince her to take the test but she won't until her next period and only if she misses it. Her pregnancy symptoms are: Cramping, fatigue, mood swings, temperature change, backache, constant urination, spotting, discharge, hungry more often then usual, dizziness, metallic taste, nausea, foggy memory, and a few more but those are right off the top of my head. I understand some are signs of a period (her period should begin on Monday if she isn't pregnant) but she rarely will experience cramping, bloating, or mood swings/PMS. Could she be pregnant? If so, How can I convince her take a pregnancy test?