Hello Bug!
I suspect your H is motivated to act (enable mostly) based on long held family patterns. I wonder if you asked questions about how his family functioned, the alcoholic mom (now deceased), and your H's feelings and roles he was required to play growing up might be a small window into why he now is enabling BIL somewhat behind your back.
This might be a slightly more empathetic approach with your H as you want him to be open and communicative. Since BIL is about to head off to rehab, you could space out several conversations to better understand why your H probably feels deeply obligated to care for his brother. You are about to get a break. He's. Not the first addict to enter rehab with recovery NOT the main goal!
There are two things that helped me. One is realizing both me and my H are ACOA (adult child of alcoholics). For me,I fit due to dysfunctional family not straight up addiction. Your H may have set patterns, behavior and beliefs due to being raised in his family. Secondly I've had some luck recognizing when my H and I start playing out old arguments and we take our preferred corners. Our arguments are well worn angles on the Karpmam Drama Triangle.
Survivors Library: Karpman Drama Triangle
some of this history you surely know, but you've probably not looked at it from a family system of addiction. You might hold a lot of information that might help you best form a more solid unit with your H to deal with his brother. Best!