COgirl - frustrating! Ugh, I have a sister with whom I can only spend a very limited time, and I have to watch what I say when I'm with her. This is sad, because our immediate family is down to just three of us in the last few years, and there are only a few extended family members.
I've got two ideas of how you might tackle your situation. One has to do with your immediate reaction, and the other is a suggestion for accessing some possible deeper emotions.
Anger is actually a rather shallow emotion. It is, in fact, sitting right there in front of your brain, firing your amygdala, and producing the equivalent of the fight-or-flight response. This is why you know it's shallow. Anger keeps you on your toes, it keeps you on the alert for danger. The emotions that your brain needs to repress are much deeper, because they are liable to distract you too much so you're not on the lookout for the sabre-tooth tiger.
This is the first thing: your primitive brain can't tell the difference between a sabre-tooth tiger or obnoxious family members! So when you have to interact with these people, your brain responds in the only way it knows how - and being in FoF constantly, especially with people who are an intimate part of your life - who should be people that you want to love and who love you - is very tiring to your poor nervous system. Knowing this, and changing your inner reaction in order to protect your nervous system, will go a long way to soothing you physically as well as emotionally. It's all about your inner dialogue.
This is something I've really learned the last two weeks thanks to the Mindfulness Summit!
The other thing that occurred to me, literally as I was writing that first sentence about my own sister, is the sadness of never having had a close relationship with her. In your case, to add insult to injury, your mother is being manipulated by your sister, and I have to wonder if this combination results in a dual sense of loss for you.
Because here is the second thing: Isolation and abandonment are really important emotional issues for us humans, and they are typically quite deeply repressed. Between your useless sister, enabling mother, and irritating in-laws, how could you not feel isolated and abandoned, especially at this very vulnerable time?
I don't suggest forgiving anyone other than yourself. And watch out: you might have all kinds of intellectual reasons why you are not isolated or abandoned, but your inner child is not intellectual, and she needs to be nurtured and comforted!
~Jan