We are missing some information.   Did your "significant other" have income and file a tax return?   If so, then your SO is the one who should claim the mother as a qualifying relative.  If your SO did not have any income or file a tax return, and you supported the mother, then the mother could be your qualifying relative dependent IF she lived with you for the entire year and she had less than $4300 of income not counting Social Security.    For a qualifyng dependent claimed on a 2020 tax return you can get the $500 credit for other dependents and the 3rd stimulus amount.   You cannot get the 1st or 2nd stimulus payments for a dependent over the age of 16, but the age limit changed for the 3rd one.

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  

Be aware that claiming your SO's mother does not qualify you to file as head of household since the mother is not blood-related to YOU.  

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

Your SO's mother * might*   be a qualifying relative that you or your SO can claim.

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, fiance (etc.) as a dependent on your 2020 taxes as long as they meet the following requirements:

Qualifying child

•They are related to you.

•They cannot be claimed as a dependent by someone else.

•They are a U.S. citizen, resident alien, national, or a Canadian or   Mexican resident.

•They are not filing a joint return with their spouse.

•They are under the age of 19 (or 24 for full-time students).

• No age limit for permanently and totally disabled children

  •      They live with you for more than half the year (exceptions apply).

Qualifying relative

•They don't have to be related to you (despite the name).

•They cannot be claimed as a dependent by someone else.

•They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.

•They are not filing a joint return with their spouse.

•They lived with you the entire year.

•They made less than $4300  (not counting Social Security)

•You provided more than half of their financial support. More info

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them.

Related Information:

•Does a dependent have to live with me?

•What does "financially support another person" mean?

•Can I claim a newborn baby?

One other thing----you said "we" might claim her in the upcoming year.   If you are not legally married there is no "we" for your tax return.   You cannot file a joint return.  If the mother can be claimed then only ONE of you can claim her.

I understand I wanted clarification we are not Married.   I will have my S/O look into it more. And no she has not filed taxes in 3 yes she has no tax sale income no ssi or anything.  Thank you for the info.  

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@707ink wrote:

I understand I wanted clarification we are not Married.   I will have my S/O look into it more. And no she has not filed taxes in 3 yes she has no tax sale income no ssi or anything.  Thank you for the info.  

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In general, the rules for claiming an adult will be the same in your situation; whether you or your partner should claim the mother requires some more strategic thinking.

Your partner can claim their mother if these tests are met:

• the mother's taxable income is less than $4300
• the mother does not file a tax return, or only files a tax return to claim a refund of withholding and claims no other deductions or credits
• your partner provides more than 50% of the mother's financial support

For you to claim the mother there is an additional requirement that she live in your home for the entire year (because she is unrelated to you).

In actual fact, only one person can claim the mother because only one person can provide more than half the mother's support.  In practical terms, as long as you and your partner mingle your finances, then you can probably pick which one claims the mother as a dependent by saying that person pays "more than half."  (If you each paid exactly half her expenses, then no one can claim her because exactly half is not more than half.)

The tax benefit for 2021 of claiming an adult dependent is a $500 credit for "other dependent" plus a $1400 stimulus rebate, unless the mother already received a stimulus check.  Additionally, the person who claims the mother as a dependent could also take a deduction for the mother's medical expenses if you or your partner actually pay them.  

I can't think of any other benefits of claiming a parent as a dependent.