Just take a deep breath and take it a step at a time. Also keep in mind that the usually measure the ESTIMATED GFR (eGFR) through the simpler test. If there is a problem they can also do the more complicated and expensive test to find the REAL GFR, so even if the initial results are problematic it may not be the end of the line. But the tests are to make sure that you can pretty safely donate a GOOD kidney to your sister, and still have reasonable hopes for a normal life with your remaining kidney. No sense donating a kidney and then ending up on the waiting list yourself, so be thankful that the hospital is being careful for you both.
You might also want to post at the FaceBook page of Living Donors on Line, as many more donors check in there rather than here these days.
Happy Thanksgiving!
Fr. Pat