Adoption laws vary from state to state. It is possible to adopt a family member. There are many issues involved, obviously, and a lawyer would be necessary. We're not going to get into the nitty gritty of relative adoptions because of the complexity involved, and also your question is easy to answer from an immigration perspective from the information you provided. Even if the aunt was otherwise eligible to adopt the niece, she would not be able to obtain a green card for her. Adopted children can be sponsored by U.S. citizen parents for legal residence, but one key requirement must be met before this can happen - the adoption must have taken place prior to the child's 16th birthday. As the niece in question is approaching 18, she will not be able to obtain a green card through her aunt. Also, when an adopted child obtains a green card through a parent, the child must wait five years before applying for U.S. citizenship. So even if the niece was able to obtain legal status here through the aunt, she'd still have to wait those five years before citizenship could happen.

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