My sister and her ex husband are claiming her children for the entire 2024 tax year when I had custody for 222 days. They have already filed. What do I do now?

You do not have to wait for them to amend.  That will not help you.  The children's SSN's are already the system.

Prepare your own return claiming the children who were with you for more than half the year and mail your return.  Mailed returns are processed differently, so go ahead and claim the children. Print, sign and date you return in ink.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.

Your sister and her spouse still need to amend---you just do not have to wait for that.

You can still include the children as dependents on your tax return but you will have to print and mail your tax return, regardless if they amend their tax return.

See this TurboTax support FAQ for how to print and mail a tax return when using the online editions - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turbotax-online/L5bwuqXGy_US_en_US?uid=m7w7asgm