This is all attention-seeking behaviour. Even their enabling of your wife's brother's self-destruction. Do you think they were getting nothing out of looking after their "poor, sick son"? Now that they can't milk that shit show anymore, they're taking the show out onto the road. They don't want therapy because they know exactly what they're doing. The one-hour screed at your birthday party was 100% pure and unadulterated attention-getting behaviour. Thank god your mother put her foot down.

The only solution is to stop giving them attention. This is what your wife's siblings are doing. They have stopped playing the game. This is what you and your wife need to stop doing. As soon as everyone stops doing for them what they can do for themselves, they will have to step up and do it themselves. This is not healthy for your wife or you, individually, and it won't be healthy for your marriage if your wife continues to engage them. Hopefully, when you refuse to participate in the pity parties, your wife will be empowered to do the same.