Deductions & credits

To claim your wife's mother as a dependent, she must meet the four tests of a Qualifying Relative. These tests are as follows:

1. Not a qualifying child test: She cannot be considered the qualifying child of any other taxpayer.  In your situation, this does not apply - it applies more to when there is a potential dependent that is under age 19, under age 24 and a full-time student, or permanently and totally disabled.
2. Member of household or relationship test: To meet this test, a person must either live with you all year as a member of your household, or be related to you as follows:
   • Your child, stepchild, or foster child, or a descendant of any of them (for example,
     your grandchild). (A legally adopted child is considered your child.)
   • Your brother, sister, half brother, half sister, stepbrother, or stepsister.
   • Your father, mother, grandparent, or other direct ancestor, but not foster parent.
   • Your stepfather or stepmother.
   • A son or daughter of your brother or sister.
   • A son or daughter of your half brother or half sister.
   • A brother or sister of your father or mother.
   • Your son-in-law, daughter-in-law, father-in-law, mother-in-law, brother-in-law, or sister-in-law.
3. Gross income test: the person's gross income (for tax year 2024) must be under $5,050 (note, this amount changes every year).
4. Support test: You must generally have provided more than half of the person's total support during the year.

Additionally, you must meet the three general Dependent Tests as follows:

1. Dependent taxpayer test: If you can be claimed as a dependent by another taxpayer, then you cannot claim any dependents.
2. Joint return test: You generally can't claim someone as a dependent if they file a joint tax return with their spouse.
3. Citizen or resident test: The person you are claiming must be  U.S. citizen, a U.S. resident alien, a U.S. national, or a resident of Canada or Mexico.

If your mother-in-law meets all these tests, then you would be able to claim her as a dependent.  You must show the person's social security number on your tax return, but you mentioned that your mother-in-law did receive a social security number in 2024.

Further, now that she has a social security number, you can amend prior year returns to claim her as a dependent, as long as you check the rules and make sure she met all of them for the year you are amending.

For more information, IRS Publication 501, Dependents, Standard Deduction, and Filing Information, is a great source! You can find this publication at this link: IRS Publication 501