This is a time sensitive issue that you must retain counsel on today. Your marriage and her age combined can actually reduce the issues dramatically. She's going to need to obtain LPR status before she can apply for citizenship. Clearly her manner of entry is of issue and she will have to depart the U.S. and return in order to obtain proper status. But with her being so young, there will likely not be a waiver necessary. Please get to a lawyer today.
Meet with an attorney as soon as possible. She may not have any issues whatsoever depending on her manner of entry. Even if she came illegally she may be able to avoid the necessity of a waiver for unlawful presence if she departs the country soon enough. A lawyer will be able to review all of the pertinent details and advise you accordingly.
Samuel Ouya Maina, Esq. 415.391.6612 email@example.com Law Offices of S. Ouya Maina, PC 332 Pine Street, Suite 707 San Francisco, CA 94104
If your fiance is in the US without a visa she may not be permitted to obtain permanent resident status in the US but will need to depart the US as a part of the process. If she is under 18 1/2 she may not have any unlawful presence if she departs before she turn 18 1/2 and she should be permitted to return without need of a waiver. While you can marry, I strongly encourage you and your fiance not to go forward with any applications to CIS or for your fiance to depart the US without first meeting and discussion your options with an immigration attorney.
From the information you have provided, your fiance is likely eligible for tow new processes available to foreign nationals . Deferred action and the new provisional waiver. Both are good means to permanent resident status for your fiance, couple with a petition from you.
Also if she did enter the US with a visa, the process is much more straight forward and less time urgent.
Your circumstances fit a special provision for members of the military. Please hire an immigration attorney who understands the "Parole in Place" (PIP) process. Your case fits within the guidelines of PIP and your new wife will have permanent resident status within a few months after filing if not a shorter time period. You will need to show that if she is not given status your military duty could be compromised and the military objective threatened. These cases are somewhat unique so find an immigration attorney who has completed the process successfully.
Good Luck to you and thank you for your service.