Additional information, such as where you and your mother are currently living, where the marriage is expected to take place, etc. are needed to answer your question. You should contact an immigration attorney for a thorough consultation on your case.
No. I am afraid you do not qualify for a green card due to your mother's upcoming marriage to a US citizen. To qualify, step children of U.S. citizens must be younger than 18 years of age prior to the parent's marriage to the US citizen.
Sorry, no, you are too old to qualify under your mother.
Franco Capriotti - Senior Immigration Counsel
CAPRIOTTI INTERNATIONAL LAW
firstname.lastname@example.org -- 1-503-803-0055 -- www.capriotti.com
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IMMIGRATION LAW PROFESSOR for 10 years -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.