You may be able to claim your mother-in-law as a dependent. She would need to qualify as a dependent under the rules for a qualifying relative dependent:

The rules are as follows:

1. Not a Qualifying Child - One of the first requirements for qualifying child is a certain, defined relationship. Mother in law is not a qualifying relationship to be a qualifying child dependent. Therefore she would pass this test.

2. Member of Household or Relationship Test - To meet this test, the individual must meet one of the following:

1. Live with you all year as a member of your household, or

2. Be related to you in one of the ways listed under Relatives who don't have to live with you . Mother-in-law is one of the relationships that qualify without living with you for the entire year.

3. Gross Income Test - She cannot have more than 4,050 dollars of income. This doesn’t include social security income, but it would count retirement income reported on a form 1099-R, investment income reported on forms 1099-INT, DIV, or 1099-B. The total of her non-social security retirement income would need to be less than 4,050 dollars.

Support Test - You would need to provide more than half of her support. Fill out worksheet 2 on page 16 of IRS Publication 501 to make sure you meet this requirement.

You disabled MIL claims her own income on her own tax return. Or are you asking about claiming your MIL as a dependent on your tax return?

"....collects social security and retirement.."

If your MIL had more than $4150 of income from her retirement account--do not count Social Security, though--you cannot claim her as your dependent.

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent 

Yes, I’ve claimed her before but not receiving anything!  She doesn’t claim anything and haven’t for years!  

@BeckyCUSoon1 If you can claim an adult dependent you can get the $500 credit  for other dependents.   You do not "receive" anything.   The credit reduces the tax you owed.  However, for your 2021 return, you can get the $1400 recovery rebate credit for claiming a dependent.   Answer the questions about the recovery rebate credit in Federal Review and the credit will show up on line 30 of your Form 1040.

You say she gets Social Security and "retirement"----the Social Security does not count, but the "retirement" income (pension---retirement from an IRA, etc.?)   counts.   If she receives more than $4300 of income besides Social Security you cannot claim her.

CREDIT FOR OTHER DEPENDENTS

https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...