I don't see a problem. Why doesn't the sister simply get a passport and buy a ticket? If there is an issue you don't mention (e.g., the sister lives in a country with no diplomatic relations with the US), you should see an immigration attorney.
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It depends a great deal on whether the U.S. requires a visa from your sister to visit. If she is not a citizen of the few countries that don't require a U.S. visa, then she will need to apply for a B2 visitor visa and an interview at the U.S. embassy or consulate will be required. During the application process, she will need to provide evidence of ties to her home country to convince the consular officer that she does not intend to immigrate. Evidence that she owns property at home, is leaving behind close relatives, or has other reasons not to emigrate from her home country is all helpful. Keep in mind, the fact that you and your wife are U.S. citizens will only count against her because it may appear to the consular officer that she intends to immigrate as well. Whatever she can do to downplay that aspect and emphasize her ties to home will help her get the visa approved.
You can find the procedures to apply for a tourist visa at the US Embassy or Consulate serving your sister in law's country at: http://www.usembassy.gov/ .
The fees to apply for a tourist visa is about $131. If your sister in law is a citizen of a country in the Visa Waiver Program, she likely can just buy an airline ticket and fly to the US, unless she had been denied a visa or entry into the US before.
The most important thing your sister in law must prove to the consular officer is that she will leave the US when her authorized stay is over. The website should have some suggestions as to how to prove her nonimmigrant intent.
If the sister in law is young, single, and without a career or properties, she likely fits the profile of someone who intends to immigrate to the US and will be denied a tourist visa.
As the information from the consulate should make clear, the only person who decides whether an applicant gets a visa is the consular officer. Invitations from US relatives, guarantees by prominent individuals, or promises by hustlers have no bearing on the consular's decision.