How do i claim my son stimuls payment who was born in 2020. I claimed my sister last year so its not letting me claim it for him

You should get the Recovery Rebate Credit for your son as long as he is not a dependent on someone else's return and is is considered your qualifying child. Claiming your sister as a dependent for 2019 would not prevent you for the credit for your son on your 2020 return.

Per the IRS, to be a qualifying child of an individual for purposes of the payment, generally the individual must live with the child for more than half of the tax year, the child must not provide over half of his or her own support for the calendar year, and the child must not file a joint return for the tax year. The child also must be the individual’s child, stepchild, eligible foster child, sibling, grandchild, niece, or nephew.  In addition, to be claimed as a qualifying child, the child must be a U.S. citizen, permanent resident or other qualifying resident alien. The child must be under the age of 17 at the end of the year for the tax return on which the IRS bases the payment. Also, a qualifying child must have an SSN valid for employment or an adoption taxpayer identification number (ATIN). A child who has an ITIN is not a qualifying child for this payment.

In addition, your Recovery Rebate Credit amount will be phased out if your adjusted gross income for 2020 exceeds $150,000 if you are married filing a joint return or filing as a qualifying widow or widower, $112,500 if you are using the head of household filing status, or $75,000 if you are using any other filing status.