The ICE approaches these cases as if the marriage is fraudulent, (BECAUSE there really are a TON of them that are fraudlent) and you will have to prove that it is legit. Proof are things like mortgages together, (or Leases), joint bank accounts, joint ownership of assets (Vehicles). Bills in both names,...photos, cards....letters...
You will also have to sign an affidavit of support, showing that you are capable of supporting her, and also gaurenteeing that you will support her, no matter what your relationship becomes.
The process is very expensive, cumbersome and takes a lot work. So not only would you be taking a chance at committing fraud, because of the difficulty and legal risks, I definately would not recommend it unless you are truely in love.

HOWEVER, Your first sentence indicates her parents are US Citizens??? That is the most obvious way to go about this. Unmarried child under 21 of a US citizen do not have to wait for a visa number to become available to start the process. See the quote below and also the link to the website about immigration through a family memember.
All she would have to prove is that she IS related. IF there is a concern about the parents meeting the criteria for affidavit of support, they can get a co-sponsor. You could help out that way, only taking a financial risk, not a legal one.



""The immediate relatives of U.S. citizens, which includes parents, spouses and unmarried children under the age of 21, do not have to wait for an immigrant visa number to become available once the visa petition filed for them is approved by USCIS""



http://www.uscis.gov/portal/site/usc...0045f3d6a1RCRD