Hope things go well for your sister and your niece.

In a baby this young, fevers are almost always the result of a viral infection, and are self resolving. But because on rare occasions they can be bacterial infections, and much more serious, there is a standard set of tests that we do to make sure we aren't missing something. These include a chest x-ray (to check for pneumonia), a urine test (to check for urinary tract infection) and a lumbar puncture (to test for meningitis).

And yes, taking blood from a three week old can be very challenging.