If you are a U.S. citizen with brothers or sisters who are natives of a foreign country, you have the power to obtain a green card (lawful permanent residence) for them, along with their spouses and unmarried children under the age of 21. You must be 21 years of age or older yourself. You must also be willing to help guarantee that your sibling and family will be financially supported in the U.S. and not require any need-based government assistance.
But there's a major catch: Because of limits on the number of visas given out in this category every year, there's an extremely long waiting list, as described below.
If you are not a U.S. citizen, but a permanent resident of the United States (with a green card), you cannot bring a sibling here legally. If you're interested in assisting a sibling in this way, you should apply for U.S. citizenship as soon as you're eligible.
The sibling of a U.S. citizen is at the bottom of the family preference list, in the family fourth preference (F4) category. As soon as you start the immigration process for your sibling, by filing a visa petition on Form I-130 (issued by U.S. Citizenship and Immigration Services, or USCIS), your sibling will be placed on the waiting list for a visa.
But because so many people have already applied under this category in past years, the wait is extraordinarily long -- typically at least ten years for people from most countries. Also, because per-country limits apply as well, applicants from certain countries, namely Mexico, India, and the Philippines, typically wait even longer, sometimes up to 25 years. And even after the wait is over, getting the actual immigrant visa and green card can take several weeks or months.
See below for more on how this long wait plays into the application process.
If you meet the requirements and wish to bring your sibling to the United States, you will (near the end of the application process) need to show that your household income exceeds 125% of the poverty level in the U.S., meaning that you can financially support your own family as well as your sibling's. You will do so by filling out an Affidavit of Support on Form I-864, which is essentially a contract with the U.S. government. If your sibling does end up claiming needs-based government assistance, your having signed this affidavit allows the government to come to you for reimbursement of these amounts.
To start the process, you'll need to file a visa petition on USCIS Form I-130, together with a fee, proof of your U.S. citizenship, and proof that the two of you are siblings. That last requirement generally involves making copies of both your birth certificate and that of your sibling, showing at least one parent in common. If you or your sibling has made a name change, you will also need a document that proves it, such as a marriage certificate.
If you and your sibling are related only through your father, you will also need your father's marriage certificate from his marriage to your mother, as well as the one from his marriage to your sibling's mother. You will also need an annulment or divorce decree, or a death certificate so that it is obvious that the first marriage ended legally. If the parents were never married, you or your sibling will need evidence of legitimation, or proof that a financial or emotional relationship was established with the father at some point before age 21.
After USCIS has approved the visa petition, your sibling will receive a "priority date," based on the day USCIS first received your petition. Then the long wait begins. Advise your brother or sister to warn their children that, if they want to come along on this visa, they must not get married before entering the United States. (Turning 21 can also make them ineligible, but they have no control over that.)
You'll need to start tracking the progress of priority dates in the fourth preference category. You can do so by monitoring the State Department's Visa Bulletin. When the dates shown on the family-based visa chart for the 4th preference category start to get close to that of your sibling's, then start looking for letters coming from the National Visa Center (NVC). Get in touch with the NVC if you start seeing priority dates on the chart later than your siblings -- perhaps you forgot to send them a change of address and their letters went astray.
If by some unlikely chance your sibling is in the U.S. on a valid visa when the priority date becomes current, he or she should be able to adjust status here (get the green card without leaving to process through a U.S. consulate). But that doesn't mean it would be a good idea for your sibling to get a tourist visa to come here right before the priority date becomes current, with the hope of adjusting status -- that would constitute a fraudulent use of the tourist visa, and potentially lead to the green card being denied.
More likely, your sibling will do "consular processing," by communicating with, and attending a visa interview at, a U.S. consulate in his or her home country. If all goes well, he or she and family will be issued immigrant visas to the United States. Upon entering the U.S., they will become permanent residents, and receive their actual green cards a few weeks later.