I'm trying, but (as I'm sure you know) that doesn't mitigate the frustration. Some of it's because of his sister, but another large chunk is because he no longer gets to see his mom (my ex) on a regular basis. This is because she remains incapable ob being a functional human being and was kicked out of the place she was living with a guy who was the most decent thing that had happened to her in a while. It turns out that - among other things - he got fed up with her not working but wanting to live a certain lifestyle. Once again, her inability to make progress as a human being is causing everyone - especially her son - harm, which leads to to greater stress on all of us. I'd be pissed off about this in general, but this is the third time this has happened.
The thing that did piss me off tonight though was when she called and I had to tell her that he couldn't talk to her because of his behavior. Her response was that it's because he doesn't get to see her and this would be fixed if she could spend more time with him - completely ignoring the fact that the reason he isn't spending time with her is because of her own actions. That still gets under my skin.