You can never claim EIC as the non-custodial parent. And if the ex-spouse or a court says you can, that's invalid since a state court can't override federal law.
The dependent splitting rules for children of divorced or separated parents are in publication 501 <a rel="nofollow" target="_blank" href="
https://www.irs.gov/uac/about-publication-501">https://www.irs.gov/uac/about-publication-501</a>
In brief, the custodial parent automatically gets all tax benefits of a dependent,
1. exemption
2. child tax credit
3. qualify for head of household
4. earned income credit
5. child and dependent care credit or dependent care FSA
The non-custodial parent can claim the children as dependents if the custodial parent gives the non-custodial parent a signed form 8332, or if there is a divorce decree from 2008 or earlier that has certain narrow and specific qualifying language.
Even if the non-custodial parent can claim the child as a dependent, they can only get the exemption and the child tax credit (#1 and #2). The other benefits always stay with the custodial parent and can't be waived or transferred.
In Turbotax, if you answer the dependent interview by saying that you had custody less than half the year, and are claiming the exemptions because of a form 8332, Turbotax will only give you #1 and #2 and will not allow you to claim the other benefits incorrectly. However, if you tell Turbotax you had custody more than half the year, the program won't know any better and you will get those other benefits, and you may end up being investigated by the IRS and required to repay some of those tax benefits.
If you have a form 8332 signed for "all future years" then you should have no problem claiming your children as dependents as long as you indicate in the program that you have custody less than half the year, so that you only take #1 and #2. Your ex and her new spouse can claim the children for EIC, but if they try to claim the children as full dependents, both of you will get letters as part of an IRS investigation. You would show proof by giving the IRS a copy of form 8332 -- which you are required to mail with your tax return anyway.
If you are being blocked from e-filing because your ex has already filed, and you believe you have a legitimate claim, print your return and mail it in.
Also be aware, your ex can TAKE BACK the dependent waiver (cancel the "all future years" choice) by giving you another signed form 8332 using part 3 of the form to indicate that she is taking back the exemptions. However, this does not take effect until the tax year AFTER the calendar year in which she gives you the new form. In other words, if she gives you a new form 8332 any time in 2017, you can still claim the children as dependents in 2017 but not in 2018 or after.
