I'm not quite sure why you're lumping your resentments about your SIL with your resentments about your husband. They are, individually and collectively, a mess, but one has little to do with the other's alcoholism. Someone who's close to her might conclude that your husband is a bad influence on her. That would be equally incorrect.
I think alcoholics in denial do support each other's denial and enable each other. I'm sort of surprised you expected her to stand up for his sobriety and your marriage when her life is a train wreck on its own.
And, I'd also point out, she could be a model of mental health and your biggest cheerleader, and she STILL wouldn't be able to get him sober--any more than you could. So I think there's a bit of misdirected anger here.