She will not go to jail. Since she is under 18, the IRS will likely question her "independence" and adjust accordingly. So wouldn't file an amended return just yet.
No your sister will not go to jail for a mistake on an income tax return. BUT.....if she can be claimed as someone else's dependent (parents) then she cannot receive that stimulus $1400 and if she did get it back as part of a tax refund she has to pay it back to the IRS.
She (or you--since you are the one who prepared the return) needs to prepare an amended return and change the answer in My Info to the question of if she can be claimed as someone else's dependent. She has to say YES that she can be claimed. She will have to pay back that $1400 recovery rebate credit.
https://ttlc.intuit.com/community/tax-return/help/how-do-i-amend-my-2021-return/00/2544441
Do not amend until the original return has been fully processed.