Did your son-in-law pay a professional tax preparer -----who incorrectly filed his return as Single? If so, that preparer made a big mistake and should be the one to fix the mistake and prepare the amended return. Yes----the preparer could change it to a joint return and include your daughter's information.
If they were legally married at the end of 2023, their filing options are married filing jointly or married filing separately. Filing single is not an option for either one of them.
What they CANNOT do is switch from a joint return to married filing separately AFTER April 15, 2024. Other amended returns can be sent before or after April 15. The only thing that is forbidden is to switch/amend from MFJ to MFS after the filing deadline on April 15.
If your son-n-law's return is changed to a joint return, that will take care of your daughter, too. She would not have to file her own return, since all of her information would be on the joint return.
JOINT vs. SEPARATE RETURNS
If you were legally married at the end of 2023 your filing choices are married filing jointly or married filing separately.
Married Filing Jointly is usually better, even if one spouse had little or no income. When you file a joint return, you and your spouse will get the married filing jointly standard deduction of $27,700 (+$1500 for each spouse 65 or older) You are eligible for more credits including education credits, earned income credit, child and dependent care credit, and a larger income limit to receive the child tax credit.
If you choose to file married filing separately, both spouses have to file the same way—either you both itemize or you both use standard deduction. Your tax rate will be higher than on a joint return.
Some of the special rules for filing separately include: you cannot get earned income credit, education credits, adoption credits, or deductions for student loan interest. A higher percent of your Social Security benefits may be taxable. Your limit for SALT (state and local taxes and sales tax) will be only $5000 per spouse. In many cases you will not be able to take the child and dependent care credit. The amount you can contribute to a retirement account will be affected. If you live in a community property state, you will be required to provide additional information regarding your spouse’s income. ( Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI)
If you are using online TurboTax to prepare your returns, you will need to prepare two separate returns and pay twice since with online, you get one return per fee.
https://ttlc.intuit.com/questions/1894449-married-filing-jointly-vs-married-filing-separately
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
https://ttlc.intuit.com/questions/1894449-is-it-better-for-a-married-couple-to-file-jointly-or-separ...
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
