You really do need to talk to someone, you are torturing yourself. Can you get the whole family together so that someone can mediate and you can all have your say about how you are individually feeling. When you know the worst, you can confront it head on and put it to bed. you can probably find the support in your area on this site. I take it Kim is still on drugs? It may be useful for her to talk so that she can focus on the next step after coming off drugs.
Radio 5 did a program recently where a young man, a rehabilitated drug user had started mentoring people who wanted to get clean. It is all something that can be discussed. I hope she is open to support and help. Otherwise, Making Space was an organisation I used to help me deal with my sons addictions. Whether Kim is willing or not,you need advice and support on how to look after yourself and your dependents before anymore emotional damage is done to you. My son was never willing, the situation damaged us all because I felt I could help him and it was all my or my husbands fault. You can’t keep thinking like that.
You are not a bad person, Kim will only come out of this when she is ready. Meanwhile please, please look after yourself and your other children. x