She cannot use her visitor visa to enter the US with the intent to marry and immigrate.
However if she comes as a visitor and after a period of time you and she decide to marry then there is no fraud or misrepresentation on her entry.
Matter of Cavazos, 17 I&N Dec.215 (BIA 1980)
That case held that the presumption of immigrant intent does not apply to immediate relatives if adjust ment is filed within 60 days.
However, that does not mean that the USCIS cannot look at other facts and they have many resources available to investigate whether there was a fraud or misrepresentation to the CBP when questioned at entry. So for example, if the CPB officer asked your father why he was coming and if he was going back home and he answered "yes" but had abandoned all his ties to his home country then the USCIS could consider his statements as being a misrepresentation or fraudulent. So the adjustment could be denied on that basis and not on the immigrant intent issue.
If she enters as a visitor before December and you spontaneously marry she can remain in the US while you and she go through the process to get her green card
Here is what you will need: Find instructions for each here:
I-130 Immigrant Petition (signed by US citizen spouse)I-485
Application for Adjustment of Status (signed by alien)
I-765 Employment Authorization Application (signed by alien)
I-131 Application for Travel Document (signed by alien- this document permits alien to travel during the pendency of their "green card" case)
G-325A Biographic Sheet (one for alien and one for their spouse)
A medical examination in a sealed envelope performed by a civil surgeon (find one here: https://egov.uscis.gov/crisgwi/go?action=offices.type&OfficeLocator.office_type=CIV
I-864 Affidavit of Support
Two passport sized photos of spouse
Six passport size photos of alien
Copy of alien’s passport including all blank pages and I-94
Copy of US citizen birth certificate, natz cert or passport
Copy of spouse's birth certificate
Copy of any divorce decrees
Copy of marriage certificate
One check for the I-130 in the amount of $420
One check for the I-485/765/131 in the amount of $1,070
The packet get filed at the PO box on Page 6 of the instructions
You can read about the process at this site
Conditional Resident status is granted for 2 years. You and your spouse have to file a joint petition to remove this conditional status within 90 days of the 2nd anniversary date of the adjustment approval.
If you marry in Mexico she will be turned away at the border as an intending immigrant and will need to wait there for approximately 6-9 months to get her visa to enter as your spouse under a K3
or CR1 visa. The K3 permits her to enter while your petition is still pending and the CR1 is where she enters as a permanent resident.
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