Not an immigration question.
IMMIGRATION LAW PROFESSOR for 10 years -- LEGAL DISCLAIMER: This answer is offered for informational purposes only. It does not constitute an attorney-client relationship.
There is no question that your girlfriend needs to consult with an experienced immigration attorney about the best process for getting married, obtaining permanent residence and her ability to travel in and out of the United States. That same immigration attorney ought to have some familiarity with family law in your state because you will each need to obtain a separate family law attorney if the two of you want a premarital agreement and the immigration attorney will have to make sure that the agreement comports with aspects of immigration law, such as financial sponsorship liability.
The answer provided does not create an attorney-client relationship and is only based on the information submitted in the question. If other information were submitted, it could result in a different answer. Consult with an experienced immigration law attorney for legal advice.