I recently became a U.S. citizen, and both my parents are overseas, with no U.S. residency rights. I would like to sponsor them for U.S. green cards. But they divorced long ago, and both have since remarried.
How does this work? If I petition for green cards for my parents, can my stepparents come too?
Congratulations on becoming a U.S. citizen. Your new status makes your biological parents into "immediate relatives," in immigration law lingo.
That means you can petition for your parents to obtain U.S. lawful permanent residence (green cards) without having to deal with annual numerical limits and thus long waiting periods. The procedure for doing this is to file Form I-130, issued by U.S. Citizenship and Immigration Services, after which you and they would file other paperwork with a U.S. consulate and eventually attend a visa interview. (Get detailed instructions for preparing and submitting the I-130 form.)
As for whether you can also sponsor your parents' new spouses, that's a bit more complicated.
Unfortunately, parents of U.S. citizens do not fall into a visa category that allows "derivative" visas for spouses and children.
In some visa categories, by contrast, the spouse and children of the main immigrant are "derivatives," who can basically ride along on that person's petition. They don't need their own, separate, relationship to the U.S. citizen, nor for a separate I-130 petition to be filed with USCIS on their behalf. The whole family's applications will be processed at once. Again, however, that doesn't work for parents of U.S. citizens; no one can ride along as derivatives on their I-130s.
It is possible, though, that you have a legally recognized relationship with your parents' new spouses, and can separately petition for them (file I-130 forms). It depends on when their marriages took place.
Under U.S. immigration law, a "stepparent" relationship is formed if the couple married when the child (you) were under the age of 18. (See I.N.A. 101(b)(1), 8 U.S.C. 1101(b)(1).)
It doesn't matter how old you are now, or how long ago the marriage took place. In fact, it would be impossible for you to petition a parent before turning 21 yourself. The important thing is that you hadn't reached your 18th birthday by the time of the biological parent's new marriage.
So if, by chance, both your biological parents' new marriages took place before you turned 18, you have two legal stepparents as well as two biological parents, and can file a total of four I-130 petitions. You can sponsor all of them to come to the United States and become lawful permanent residents.
The USCIS Adjudicator's Field Manual confirms, "There is no limitation on the number of parents for whom a single petitioner may file visa petitions."
Bear in mind, however, that financial requirements might create a separate limitation. You will need to show that your income and assets are high enough to support all four parents, in addition to any of your current dependents. And U.S. immigration authorities might look extra hard at your finances if your parents are elderly and in need of health coverage. See Nolo's articles on The U.S. Sponsor's Financial Responsibilities.
What if one or both of your parents' marriages took place after you turned 18? Your family would need to make some difficult choices. Assuming no other family relationships with U.S. citizens are in the picture, your biological parent would need to sponsor the other parent after coming to the U.S. and receiving permanent residence.
Unfortunately, that would be a long process. When a permanent resident files an I-130 for a spouse, it serves only to put that person on a waiting list for a visa, and the wait could be years long. The reason is that spouses of lawful permanent residents are NOT immediate relatives, but what are called "preference" relatives, with only a limited number of visas/green cards available each year. See Applying for a Green Card Based on Marriage to a U.S. Permanent Resident: Overview for more information.