My wife has custody of our two oldest girls with visitation "as set" meaning as agreed by her and I outside of court. No custody has been established for our youngest daughter. She is threatening to relocate with the girls, leaving in the next 24 hours. I have been told I need to file a modification in the county in which the girls reside, but she will be gone by the time I can do that. Is there any way I can keep her from taking my kids??
If you can't file before she's departing, no there's nothing you can do. But you should contact a local family lawyer and file a petition with the family court alleging custodial interference.
you could file for emergency relief by order to show cause. Be prepared to answer specific questions about how you know she is planning on leaving. you will need to file for custody of the youngest. I am surprised that your prior custody order does not address relocation. The biggest hurdle you have will be that you are "anticipating" iinterference. So the more concrete and detailed you can be; the better.