Between the both of your paychecks you need to have enough withheld to cover your tax liability. Right now her employer is withholding nothing is because if that was the ONLY income for the year the 2 of you have you would owe nothing.
Use the IRS tool to figure how both of your W-4 forms should be filled in ...
http://www.irs.gov/Individuals/IRS-Withholding-Calculator
