Gaining Custody of a Stepchild

My question involves a child custody case from the State of: Oklahoma

I have been with my wife for 10 years, married for 6. She had a child when we first met and I have raised him as my own. We have also had two other children. Now the boys are ages 2, 6, and 10. My wife has developed a addiction problem with pills and alcohol. She is a functional addict. She works during the weekend, but a soon as she gets off, she starts drinking. She is drunk and on pills everyday. She starts around 6:00 p.m. and is usually stumbling around the house throughout the night. She passes out on the floor, she urinates the bed, and she just withdrawals herself from interacting with the kids. She attempted rehab a couple of months ago, but failed the first urine test (pills and alcohol) and soon dropped out. I told her then that she could not stay at the house if she was drinking. She checked into a motel and proceeded to drink all weekend. She came back on that Monday and promised to go to AA and see a therapist. It has been a month and she has not done so and refuses to seek treatment. She is progressively getting worse. I constantly find vodka and wine bottles and beer cans hidden through the house. In the past, I found my six year old with a pill in his hand that turned out to be a painkiller. She takes the kids to the liquor store and leaves them in the car as she goes in. She has even done this the two year old left in the car by himself. All these experiences can go on and on, so I will get to the point.

I want to file for divorce, but I do not want to take a chance at the oldest ending up with her. I want to grant her visitation rights and want her to be fully involved with the kids. I just don't want them around her at night when she is taking pills and drinking. Her family is supporting my decision. They have witnessed her behavior and addiction. The father of the oldest never had contact with him and was killed in a car wreck a few years ago.

My question is, what do I need to do to ensure that the oldest boy does not end up with his mother? What steps do I need to take to prove that she is an unfit mother? I do not want the children to be broken up and certainly do not want them to end up in sole custody with their mother. What are my rights?

The short-term solution would appear to be to seek guardianship of the child. This will be easiest if mom consents.

Thanks Mr. Knowitall for the quick response.
I am not going to get consent from the mother. She still views herself as a great mom. This is the problem with her being a functional alcoholic. She still works, but when she is off she is drunk. She drinks herself to the point of triple the legal limit on a regular basis. Do you have any suggestions on how to prove she is an unfit mother?

Call child services.