There could be a number of reasons:
1. Your sister may have interpreted results incorrectly.
2. Very rarely test can give false results.
3. She may have very early pregnancy
, that cannot be seen by ultrasound
4. She may has an ectopic pregnancy.
Please note that clinical signs are highly unreliable & if you have doubts, you may advise her to take a blood pregnancy test. If this is negative she can't be pregnant regardless of her symptoms.
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