I am a green card holder here 8n Seattle Washington
As a legal permanent resident, you are not eligible to be approved for any family-based immigrant visa petition for either your sister or the sister's child.
Once you are a US citizen, you can file an immigrant visa petition for your sister. The son can be a derivative beneficiary of the sister's petition if the son is under 21 and unmarried when the sister is interviewed for the immigrant visa. That interview will occur between 12 and over 20 years at the current processing speed for sibling petitions.
You can review the specific facts with your attorney to find out your legal options.
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