If your sister has a dui conviction or is serving a deferral period in lieu of a conviction, she had counsel, either retained or court appointed. Any and all questions your sister may have regarding her conditions and sentence should be address to her attorney by her.
If your sister has an interlock on her car in Oklahoma, it is probably the result of losing her license through the Department of Public Safety. In Oklahoma, she would have had 15 days to contest the license revocation. If she didn't or if she did and was not successful, then she received a 180 day (six month) license revocation providing she had no prior revocations within the previous 10 years. She probably asked for a modified license to let her drive during the six months which would have required an interlock in the vehicle. Because you stated she has to have the device for two years then I am guessing she either refused the breath test or took the test (or blood test) and the result was a .15 or more. This being so, under the Erin Swezey Act which took effect Nov 1, 2011, she is being required to keep the interlock on her car an additional 18 months after she reinstates from the DL suspension. It boils down to 2 years of the device on the car. Unfortunately, this may have been preventable in the beginning if we could have fought it then but after it has occurred....there is no way to avoid the requirement now. If my answer was helpful, please select it as the must helpful. Good luck.
Answers to AVVO questions are for general purposes only and do not establish an attorney-client relationship. This answer is not a substitute for legal advice and should not be relied upon without a full consultation with an attorney that can fully look at issues involved.