What jumps out at me when I read this is; she's taken 7, not 1 or 2, but 7 tests in a 10-day span. That says anxiety and obsessing on getting pregnant. That kind of mental stress can actually make your period be delayed or skipped. You can't 'wish' yourself pregnant and periods are skipped for many reasons, not just pregnancy. Right after you stop the pill, periods are often irregular.
Your sister needs to get her mind OFF being pregnant, relax and let nature take it's course. Her focus on being pregnant isn't mentally healthy for her.
If she doesn't start her cycle in a couple of weeks, then take another test with the 1st urine of the morning. If it's still negative, then schedule an appointment with her physician or ob/gyn.
Good luck to her!