No your sister shouldn't be the reason. You might have already got the 3rd Stimulus payment yourself last year? The Recovery Rebate Credit on Line 30 is only if you didn't get the full amount or qualify for more.
Did you not get the 3rd Stimulus payment? Or qualify for more? Line 30 is only if you didn't get the full amount or qualify for more. The IRS shows they already sent it to you.
Double check your bank statements and Check your IRS account (both accounts if married)
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
