Dependants

No, your baby's father cannot claim him as a dependent.   Only the custodial parent or the custodial tax-paying adult who is supporting the child and with whom the child lived can claim the child as a dependent.

If you and your child are being supported by your sister, she should be able to claim both of you as dependents on her tax return.   If the other parent files first and claims the child --your sister's e-file will be rejected and she will have to file it by mail instead.  Then the IRS will sort out the situation and determine who could claim the dependents.

For your sister:

IRS interview to help determine who can be claimed:

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

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2024 taxes as long as they meet the following requirements:

Qualifying child

• They're related to you.
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They're under the age of 19 (or 24 for full-time students).
• No age limit for permanently and totally disabled children.
• They lived with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year (exceptions apply).
• They made less than $5050 in 2024 (not counting Social Security)
• You provided more than half of their financial support.

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them. There may be other tax benefits you can get when you claim a dependent.

Yes, your sister can claim you and your son as dependents if you meet the requirements set by the IRS.  The requirements are:

• Be a qualifying child or qualifying relative
• pass the residence test
• pass the support test

TurboTax provides the following information on the requirements to claim a qualifying relative: 

• Relatives must live with the taxpayer year round and be a qualifying relative or child.  Qualifying relatives include siblings and qualifying child includes nieces and nephews.
• Dependents have to make less than $5,050 in 2024.
• Taxpayer must pay more than half the qualifying dependents support

Your son would only be able to be claimed by one person.  The IRS does not allow a dependent to be claimed on multiple returns.  Since the father does not provide support or residency for your son he would not meet the requirements to claim your son on his return.

For more information on dependency status see the links below:

• Rules for claiming a dependent
• Publication 4491

@Cturner2004