I find it's best to ask myself 4 questions before "helping" anybody:
1. Has he/she asked for my help?
2. Is he/she doing everything in his/her power to help herself?
3. Can I afford -- materially, physically, emotionally, mentally and spiritually -- to help him/her?
4. Do I want to help him/her?
If the answer to question #1 is "no," then I am sticking my nose in something that's none of my business. (The fact that I am related to someone and/or that I care about them very, very deeply does not entitle me to interfere, uninvited, in their business...and, an invitation from another friend or family member does not make the answer to question #1 "yes." In other words, if your brother asks for your help, you might decide that you can help him with his own Al Anon / codependency issues, but that still does not consitute a valid excuse to force "help" your SiL has not asked for onto her under the guise of "helping" your brother.)
If the answer to question #2 is "no," then I am probably going to end up enabling the person and/or p*ssing him/her off rather than helping.
If the answers to questions #3 and #4 are "no," then I am not taking care of myself, in which case I am in no position to be trying to take care of anyone else, and/or I am setting myself up for some big-time resentment, which is not helpful to anyone or fair to the person I am supposedly trying to help.
Unless I can honestly answer all 4 of those questions in the affirmative, then it is most likely not going to be good for anyone involved for me to help -- in fact, I am most likely not actually going to be helping anyone, including myself. Rather I'm going to be playing some kind of martyr or savior role for fundamentally selfish and self-agrandizing reasons.
Also, I've got to say that, in family situations like this, I often find it more difficult and crazy-making to deal with the other codependents involved than with the person who is, supposedly, "the problem," because a bunch of codependents who have found a great prospect to save do not respond at all well to anyone who refuses to go along with and/or support and/or applaud all of their plotting and planning to save the day. So, if you end up having to/deciding to detach from this situation for any reason, just be aware that that might mean detaching from more than the active alcoholic.
freya