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I think from the symptoms you describe I would be very concerned. First, it is unlikely that she would "catch" anything from a human or other species like bird or cat. Second, even if she did manage to pick up a viral infection of some sort from one of your sister's dogs, these kind of symptoms are not typical.
Your description makes me suspicious that she is painful. I would be most worried about either abdominal pain or hip/spinal pain from the way you describe her reluctance to move. The panting and the decreased appetite may go along with it as well. I definitely think this is something that you should have her at least examined for. If it is truly just a viral infection, she needs supportive care, but if it is something more serious, then they can better decide what needs to be done.
If her gums are pale or tacky (not moist), I would seek emergency services, otherwise I would make sure to get her into your vet as soon as you can. I would not offer any medications at this point as it may limit what your vet will be able to offer her to help. I hope this helps and please let me know if you have any further questions.