Unlike alcohol which is physically addictive, mj at most is psychologically habitual. One can stop taking in mj without any adverse physcial effects. Either substance is mind altering and a choice but alcohol is a lot harder to quit than mj.

I self medicated with mj for many years until I had to finally admit that my over use of it contributed to my depression and disabled my own abilities to cope with stress. It is unusual in my experience for someone to use both alcohol and mj. Usually it is one or the other. It is like mixing uppers with downers.

That aside.... there is nothing you can do until your brother acknowledges he has a problem with either substance. The most you or your sister in law can do is find support for yourselves. AL-ANON comes to mind. Google it to find one in your area. It is an organization like AA where people who have family members who are addicts meet to give and get support.

Your SIL has many issues that are aggrevated by your brothers lifestyle choices and she needs professional help. The most you can do for her is to help her get the help she needs.

You can't do anything for your brother until he acknowledged that he needs help. You will learn from alanon strategies that others use to cope with living with an addict.

Good luck. You are a wonderful caring person and I wish you well.