You don't get an additional $1,400. Your Recovery Rebate Credit is based on your 2021 tax return, so you are entitled to a credit of $7,000 for yourself plus the four dependents on your 2021 tax return. That includes the credit for your new son. You are not entitled to a credit for your mother because she is not a dependent on your 2021 tax return.
The stimulus payment that you received was an advance payment of the credit on your 2021 tax return. The amount of the payment was essentially an estimate of your 2021 credit based on 2020 information. Although the amount was calculated based on 2020 information, it happens to be the correct amount for your 2021 credit, so you don't get any additional Recovery Rebate Credit on your tax return. You already got the full $7,000 credit that you are entitled to for 2021. The fact that the calculation of the advance payment was based on a different dependent does not entitle you to an additional credit.
Again, you are getting the credit for your newborn son. You are not entitled to a credit for your mother.
