This sounds to me like nothing more than an attempt to manipulate you. To what end? I dunno. At the end of the day, you have to take care of You. And if that means staying out of the unhealthy relationship between your brother and your mom, then that's what it means. Taking care of ourselves doesn't always feel good, at least, not at first. Working towards understanding what is ours and what it someone else's is a good way to diffuse the emotion around things like this.