Well, MKAC, you have a lot of stuff to be really legitimately angry about. I don't think you'd need anything more burdensome to be really angry at your little sister and brother in law. What they're doing is really despicable and dirty. Brother in law sounds like a real piece of work. Sounds to me like you're handling it as best you can, and probably better than most of us would.

But yes. I think your T is referring to the little things that blow up in ways they normally wouldn't. I'm not really sure how not to do the "It's ok, this is not a big deal, I need to calm down [stuff stuff bottle bottle] the end," but I don't know, maybe doing some kind of free association type thing? This might be hard to do by yourself, but if you just keep badgering yourself with "But why does that make you angry? But why does THAT make you angry?" maybe you'll surprise yourself.

Another thread you had started about not having anything to say about your father in law made me think of my grandmother as well, and I responded saying so. But specifically, I was also thinking of the way in which I finally did really burst into tears and cry inconsolably about it, which happened about 4 years later. I was in therapy and my T and I had been having a hard time, I felt like nothing I did was good enough for him, I was under serious pressure at work and from friends and family, and while I was talking about these things I just said, "and I just miss my grandmother so much" and just exploded into a weeping mess. My T was totally taken aback, and I proceeded to sob and ugly-cry for several minutes. When I finally calmed down enough, I was able to explain to him and to myself that my grandmother felt to me like the only person for whom I was ever good enough. She was always very happy with me just the way I was, and right then I felt like I needed that SO MUCH. I know this story isn't really about anger, but what I am saying is, the emotions of grief come out in bizarre and unexpected ways.

Be patient with yourself.