Your sister in law is still considered a permanent resident but her green card is not a valid entry document and proof of her status if she remained outside the U.S. for more than one year. The proper way would be for her to apply for a returning resident visa. Otherwise, the border officer will likely allege that she abandoned her residence when she returns and ask her to voluntarily give up her green card or refer her to an immigration judge to determine whether she indeed abandoned her resident status. Only a judge can make that determination. I strongly recommend that you consult with an experience immigration attorney to determine her options to keep her green card or reapply as the issues cannot be fully considered in this forum.
There are not enough facts here to answer the question. If your sister has a valid reentry permit, then she is ok. If she does not, she needs to obtain a special immigrant visa before returning to the US. Otherwise, it is likely that she will be detained upon arrival to the US and that she will have the reimmigrate.
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.
My colleagues are all correct. You need to contact an experienced immigration attorney for a legal opinion based on all of the facts.
Legal disclaimer: The statement above is provided by CC Abbott is based on general assistance and not intended to be a legal opinion because not all the facts are provided. The person requesting information and all others reading the answer should retain an attorney who is permitted by the state bar within the jurisdiction who can examine the complete facts and provide a legal opinion on your case. All information provided in the above answer and other information provided by CC Abbott does not create an attorney/client relationship within any state of Federal law.