Stand by your SIL. She needs your help and support. I reached out also and was told by BIL that there was no problem and my AH lies because I get so angry. My inlaws have also gone against me after speaking to my AH because I can only imagine the load of bull he told them. They have not bothered to get my side even though we have known each other since I was 12 and been very close. She needs you so much right now. Don't let her face this alone. Get your hands on every book you can involving alcoholism. Best of luck and all my prayers to you and your family. Don't forget to give you SIL a hug.
Zoe