Yes, her sister would be able to file for her green card. However, the current wait times for sibling sponsored visas is about 10 years. If it's something they would like to move ahead with her sister will need to fill out and submit Form I-130 with USCIS. She'll also need to include the following supporting documents:
1. A copy of her birth certificate and a copy of her sibling's birth certificate showing that they have at least one common parent.
2. A copy of her green card, US passport, or Certificate of Citizenship.
Note, the wait time for fiance and spouse visas are significantly shorter, ranging from a few months to about a year.
There are the other potential options of employment visas, here are the requirements and you can see if your girlfriend qualifies for one of the categories:
H1B visa: issued to a specialty occupation, requires theoretical and practical application of a body of specialized knowledge along with at least a bachelors degree or its equivalent.
EB2 visa: issued to professionals with advanced degrees and individuals with exceptional ability in the sciences, arts, or business generally requires a job offer and a labor certification issued by the Department of Labor (DOL).
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