Youre worrying yourself over nothing. Your sisters sample was fine, providing that she had not consumed an abundant amount of fluids, and that she was drug-free prior to giving you the sub sample. The fact that she is a nursing mother is totally irrelevant when it comes to drug testing. Drug testing is not CSI. Theyre not going to run your sample through some Ultrex Digitech MasterDynamic 5000 supercomputer and declare "Oh, this sample came from a nursing mother".
Drug tests are just not conducted in that manner.

Split-samples work like this: Sample is split into Bottle A and Bottle B. Bottle A is the sample that gets tested while Bottle B gets put into storage. If bottle A tests positive for a particular drug(s) on the confirmation test, and the donor decides to mount a challenge to the result, then Bottle B gets tested. Again, as long as your sub donor was indeed drug-free and not overhydrated, you have nothing to worry about.

And yes, if a sample tests negative on the initial assay, the test is over then and there as a "pass" - no lab confirmation test will be performed. It would be a huge waste of time and money on the part of both the lab and client to do so.

If you dont hear any word from your employer, call them up and ask for your "start date". Keep the convo limited to that topic and never inquire about the test results. That would just make things look suspicious if you want to be viewed as a "drug free person". Individuals who are truly drug-free never ask for their results because theres no reason to. Youd have to assume the mindset "I dont use drugs, therefore my drug test is a non-issue".