If you get married, your spouse can enter the U.S. as an H4. Her siblings cannot. You could try to file for humanitarian parole if she is the only one supporting her sister.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
Your question omitted a few key facts. First, where is your girl friend located now? Is she located abroad or is she in the United States in another legal status? If you marry, she may be classified in H-4 status upon presentation of your marriage certificate/license at the U.S. Embassy in her country or upon filing an I-539 application for change of nonimmigrant status in the U.S. with all supporting documents including proof of your marriage if she is in the United States as above. As to her sister, she would not be immediately eligible for a status derivative from your H-1b status. If you become a permanent resident (green card holder), when you marry your girlfriend she could get a green card at the same time if she is already in the U.S. in H-4 status. There may be special relief available to your girlfriend's sister in law but it would by no means be a certainty. I would suggest contacting an experienced immigration attorney who can discuss a strategy with you. Good luck!
Debbi Klopman, ESq. 398 Bergen Street Brooklyn, NY 11217 www.debbiklopmanlawoffice.com firstname.lastname@example.org 718 622 1208 This advice was intended to be general in nature and not to be taken as a legal opinion or legal advice and was not deemed to create an attorney-client relationship
I do not believe that that plan to provide any legal beneficiary visa will cover your prospective spouse's sibling for that matter.
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I suggest seeking a consultation with an attorney. If you know that your sister-in-law is going to live with you and that you will act as legal guardian, you may consider adopting her. If that were to happen, she will qualify for a H4 and later on, a green card through you.
The advice suggested here is for general information only and not to be construed as legal advice.