Just let the little angel steal your heart!

I agree!
I am glad to hear she is going to keep the baby. Did they confirm the Down's with Amnio test or with a Sonogram? This is a really scary scary story, but it has a happy ending... 13 years ago, my cousin's wife had the amnio test, the Dr.'s told them the baby definitely had Down's. Then basically advised them to abort. They couldn't do that, and switched to a more supportive pre-natal Dr.
Apparently, the amnio test has to be done during an exact window of time during a pregnancy. If you are off on your conception date by even a week, then the test can give a false positive. It turned out that was the case, and my cousins son is now a happy, healthy 12 year old. Thank God they did not abort him based on the test result, and Dr's advice!
Even if your niece does really have it, I am sure she will be an amazing person that your sister, and everyone else whose heart she touches, will cherish.