I strongly ditto springsjean's reply to you.
She is absolutely correct in her explanation and her advice for your sister to see an LLMD....and ASAP.
I am curious also as to what band showed positive on her blood test.
They all mean different things. My brother-in law only showed a positive on band 23 for example (very important band) ....and I only have tested positive for band 41. Both of us have been clinically diagnosed with Lyme due to symptoms and the positive bands we have help support that.
As Britgirl said: seeing an LLMD has been one of the best things she's done.