T.T.
I agree with your sister. With your little girl just having one soft marker and everything else is fine it really doesn't point to Down's. Our perinatologist told us that just one soft marker doesn't mean much, it's when you have several that it can be an indicator. For example, if your risk was 1/5000, one marker might change it to 1/4500, not bad odds if you ask me (my first son was a 1/49 chance).
My first son had enlarged kidneys throughout the pregnancy, another marker for Down's. He is perfectly normal in every way.
Please try not to worry, it sounds to me like all is going well. Congrats!!