You will need a separate account to prepare your sister's return online.
You cannot transfer a 2021 desktop tax return to the TurboTax online editions for tax year 2021.
You can transfer a 2020 desktop return to the 2021 online editions. See this TurboTax support FAQ for how - https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-year-return-turbotax-online/L279eUvY2_US_en_US?uid=l7cd4p6j
If you have an online account then to start another tax return for someone else you will need to create a new online account with a New User ID. Go to the TurboTax online edition website to create a new account - https://turbotax.intuit.com/personal-taxes/online/
See this TurboTax support FAQ for how to start another return using the online editions - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/start-another-return-turbotax-online/L0B7Rjkyo_US_en_US?uid=l7cd833x
@jmac12 curious, why would you want to move from CD to online, since yuou have already paid for the CD (and can do multiple returns) and will have to pay again for online - once for each return.