Hi,
Welcome.
Sad thing is , there is nothing you can really do for him or say to him.
This has to be his choice to seek help.
You and your SIL should look for the nearest Alanon meeting to go to.
Even after the 1st one, don’t quit.
Most don’t they need or like it.
People seem to think there is a fast fix here that they can control, which is farthest from the truth.
I know this might not be what you were looking for, but support and knowledge of what will happen is the best anyone can do for your family.
Read, listen and learn.