Your sister in law can and should get to alanon - as soon as possible!
If she cannot get there physically, there are online groups that can help.
And there is a great book that she should read - Co-dependent No More, by Melody Beatty. She can get it on Amazon.com for cheap. Actually, it might do you some good too, as you could learn some boundaries.

There is nothing you can do for your brother until he admits to a problem. And that probably won't happen until there is a negative consequence as a result of his drinking and using.

If your sister in law is being abused, she needs to get out of the situation. NOW! There are shelters that can help her in the immediacy and can help her make plans for long term. The longer she stays, the more likely she will become one more statistic. And with the introduction of guns into this situation, it's already escalated into a horrifying situation. She needs to get out NOW!

Please voice that reality to her. But, be aware, you cannot *make* her do anything. She's an adult and she is responsible for her own actions. Once you give her helpful advice, you need to Let go and Let G*D take it from there.

I wish you well. Good luck.

Peace!