A U.S. citizen can petition his or her adult siblings regardless of their marital status.
However, because the wait time is so long, your siblings should consult with an immigration attorney about visas. With a visa, they can remain in the country while they wait the decision on the residency.
Yes, but it will probably take over 20 years for them to immigrate.
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.