The first thing to do is to talk to a neurologist. Before assuming that this is a psychological problem, it’s important to rule out the possibility that he can’t help it. You didn’t mention where the tumor was located in your brother-in-law’s brain. Depending on where it was located, there could be brain damage that accounts for some of his behavior. It sounds like his executive function may be seriously impaired.
Unfortunately, doctors don’t always talk to each other and they often only get the information that the patient chooses to give them. See if there is a way to make sure the psychiatrist is aware of his medical history and aware of his current behavior. (You didn’t mention how you are related. The closest relative is often someone who can get a patient to agree to be accompanied to an appointment.)
If he is medically clear, then it’s possible that he is so deep in his addiction, he can’t get clear of it. Sadly, AA doesn’t work for everyone. For some people, the emphasis on spirituality makes it a non-starter. Further, since it is a totally volunteer organization, the quality of the group – and the help – is dependent on who is in it at the time. Some groups have members who really work well together and work the program. But sometimes groups become just a forum for mutual complaining. I suggest you talk to your doctor about whether there is a group with another approach in your area or if there is an AA group that is known to be seriously down to business. If there isn’t an alternative group, there may be a psychologist or addictions counselor in private practice who has a different approach. Of course, your brother-in-law may not go, no matter what you say.
Meanwhile — DON’T get in a car with him. No one should! Tell his doctor what he’s done while driving. It may be grounds for losing his license.
Encourage his wife to see a counselor. If she is putting up with his behavior out of love and her own gratitude for his recovery from cancer, she may be inadvertently enabling his alcoholism. She needs support to take care of herself and her kids. She may not be able to help him but she can protect herself and her children.
This is a terrible situation. It’s terribly sad to lose someone like this. All you can do is the best you can do. Love him the best you can but stay safe yourself and encourage others to do the same.
I wish you all well.