If you are unmarried, you can be included on your parents' petition as long as you're under 21. If you turn 21 before the petition is finished, your sister might have to petition you separately, which will take much longer.
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You cannot be included (and benefit from) your sister petition for your parents, since the siblings category is separate and distinct from "parents of a US citizen", which has immediate priority and no waiting time. By contrast, the brother or sister category waits between 10-14 years to become current.
Ask your sister to immediately fie a separate petition for yo, to establish your priority date.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
Your parents may also apply for you once they become legal permanent residents (receive their green card), as long as you're not married. A petition from them would resolve faster than one filed by your sister.
[This answer is for general purposes only and does not establish an attorney-client relationship.]