See if your father-in-law to not file her taxes. It is better that you file Married Filing Jointly with your new wife on the joint return. If father-in-law files her taxes only, you and him will have to file the tax returns Married filing Separately - this wouldn't be the best way to file your tax returns. Unless father-in-law is filing for the both of you then you can still file Married Filing Jointly on that return. Also, you might want to find out why he wants to file her tax return. Her income has to be filed on one return, doesn't matter if she didn't work after you got married, her return needs to be filed with all her income. If father-in-law files her return, you can't put her on your return at all.
