Hi not really sure if I can help, but I do kind of know what you are going through
Over the past 6 months on and off, my little girl has had constant so called viruses and high temperatures. I have taken her to all different doctors and fobbed off with its just a "virus" I have finally found a doctor who knows what he is doing and it has turned out that she has a UTI ( urinary tract infection )
I am as we speak awaiting lab results and am facing having to take her for ultrasounds of her kidneys and liver etc. I am just so ANGRY that every doctor just automatically says virus. Now I am terrified that because of their incompetence that my baby may have had this UTI indefinately which could have caused kidney damage. I assume when your niece was in the hospital they checked her for UTI's ? I just thought I would mention it to you as with all of the research I have done, when children have high temps without any kind of other symptoms its almost always a UTI. Anyway good luck and I hope they come up with some answers for your family very soon.