Check her return again and see if the box for being claimed as someone's dependent is really checked --- it is on the top section of the 1040 near the words "Standard deduction." If she has that box checked then you should not be having a problem with e-filing your own return claiming her as a dependent.
If she does have it checked, then somehow her SSN has been put into the system by "someone" either accidentally or on purpose. If so, the only way those tax returns can be filed is by mail. The IRS will sort out the SSN issue.
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.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
