My sister was wondering how can she file me in her taxes if she filed already

[xmasbaby0]

Why?   Did she pay for over half of your support in 2020?   Are you her dependent?  Are you an adult who had less than $4300 of income in 2020--not counting Social Security?   Or are you a full-time student under the age of 24?    Did you have income in 2020?   Did you prepare a 2020 tax return?  

As for what your sister can do----if she already e-filed and did not claim you on her tax return she has to wait for her return to be fully processed and then file an amended return.  

Answer the questions above so we can help determine if she can claim you as a dependent.

[SweetieJean]

Or are you asking if you can file your own taxes using your sister's TT online account?  No, you must set up your own account.