Foster Son - Claimed by more then one family

No, both of you cannot claim the child as a dependent on the tax return in the same year. In order to claim a child as a dependent, the taxpayer must meet all requirements below. 

Qualifying child

To qualify as a dependent, a child must also pass these tests:

• Relationship: Be your son, daughter, stepchild, eligible foster child, brother, sister, half-sister or -brother, stepbrother, stepsister, adopted child or the child of one of these
• Age: Be under age 19 or under 24 if a full-time student, or any age if permanently and totally disabled
• Residency: Live with you for more than half the year, with some exceptions
• Support: Get more than half their financial support from you
• Joint return: Not file as married filing jointly unless only to claim a refund of taxes paid or withheld

Detailed info on each requirement can be found on IRS website

https://www.irs.gov/credits-deductions/individuals/dependents

Adopted child or foster child.

 You can treat your adopted child or foster child as meeting the residency test as follows if you adopted the child in 2023, the child was lawfully placed with you for legal adoption by you in 2023, or the child was an eligible foster child placed with you during 2023. This child is considered to have lived with you for more than half of 2023 if your main home was this child's main home for more than half the time since this child was adopted or placed with you in 2023.

Based on the info you shared the biological family would not meet the residency test for the child. You would not file an amended return. When the IRS sends you a letter asking for proof that you can claim the child, you would send back the documents requested.

Here is a link to documents you would use to prove you meet the requirements to claim the child.

https://www.irs.gov/pub/irs-pdf/f886hdep.pdf

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