It is very sad and unfair when these things come to the attention of innocent young people. This is why Congress needs to pass the Dream Act.
If your parents only recently became permanent residents, then I assume your sister also recently became a U.S. citizen. If she were not a U.S. citizen prior to May 1, 2001 or if she did not file a visa petition for you before that date, then you would be ineligible to adjust your status on any basis other than through a good faith marriage to a US citizen. If you leave the U.S. for any reason, as a 19 year old you will trigger the 10 year ban (unlawful presence began to accrue when you turned 18). Having LPR parents would qualify you to apply for a waiver of inadmissibility, but the extreme hardship standard for such a waiver is high, and is never a sure thing.
It is most important that, having lived in the U.S. for so many years and possibly having an opportunity to immigrate in the future, that you be sure to comply with all U.S. laws. I do not mean to suggest that you would do anything unlawful, but do not register to vote, vote in any election, or claim to be a U.S. citizen in order to gain a benefit. Any of these, as well as the commission of a crime (including experimental drug use) could result in your being permanently barred from immigrating.
There are many things that need to be considered in analyzing an individual's immigration situtaiton. It would make sense for your parents and you to consult with an experienced immigration attorney.
As a practical matter, you do not have to make this decision now. Your parents are LPR's (lawful permanent residents) and you are under age 21 making you a Family 2A Preference Beneficiary. You didn't list your country of nationality, but we'll assume for this answer that you are not Chinese, Meixan, Indian or Filipino. This means that you have to wait approximately another 5 years to make this decision. The Visa Bulletin published by the Department of State for the month of November indicates that F2A beneficiaries who filed in August 2005 are now eligible to initiate immigrant visa processing (either Adjustment or Consular processing). The November 2009 Visa Bulletin can be found on http://travel.state.gov/visa/frvi/bulletin/bulletin_4576.html...
For now, you should not plan to depart the United States unless you are doing so with advice of a competent and experienced Immigration attorney. Traveling outside the US will trigger the 10 year bar.
I would hope to see your case resolved by the Comprehensive Immigration Reform that the President has promised to deal with right after Health Care Reform. You can help yourself by visiting your local Congressional representative along with your sister (the US Citizen voter) to illustrate to Congress how the immigration law is hurting you and your family.
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