She won't be able to come with her parents, and although a lawyer should thoroughly check the CSPA possibility I strongly doubt she would be covered here even if listed as a derivative.
If your sister is able to emigrate under your petition and become a permanent resident, she will be able to petition for her daughter in the Family 2B category. This also has an extremely long wait time, and requires that your niece remain unmarried (this situation can get a bit better if your sister later naturalizes).
Speak with an attorney for further guidance.
Probably not. CSPA is calculated at the time the priority date becomes current.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.