I'm going to take that as a compliment.
I've had extensive discussions with my husband about such matters, and I do my best to be reasonable. I've found that unless you want someone's advice, don't tell them anything, because that validation women desire is a very rare response.
Phoenix... That's exactly true! But what I'm struggling with is what in the world can I do to let it go?
Oh, I hope you didn't take it as an insult, L.R.R.Hood! I meant that you've obviously had some male indoctrination. I find that most women like to talk out their problems, not necessarily wanting a solution (which usually they can do themselves) but wanting someone to commiserate with them, developing a common oral history as they compare their notes on life. I think somebody famous said that.
Like you, I have had extensive discussions with my husband about this, too. Sometimes, I just want to bitch about something which has already passed. Of course, with him being a man, he doesn't want to hear that. (I swear to God, they ARE wired differently.) He may (this is when we get into a disagreement) attempt to fix something which isn't fixable, or already in the past, or hassle the kids about something they did when I've already determined that next time I'll deal with it in a different way. The end result is that I don't tell him as much since he's so likely to "goober" it up in his perfectly natural response to "fix" the problem. All I want is somebody to say... "Dang, that would make me crazy, too."
As to letting it go... you could 1 - Laugh it off.
2- You could remember that "it will all be one in a hundred years" and realize that very little is truly earth-shaking.
3- You could appreciate that you even have a mum to complain about -- mine died early and I haven't had anybody to give a Mother's Day present to for 21 years. Even my sweet MiL died 15 years ago. Both died relatively young at 57 and 69.
4- You could try to "hear" their words next time and realize that they don't want a solution, they are trying to work through their own difficulties.
5- You could, as I said, stun your sister into realizing she's being an ass when she repeats negative things her husband says.
6-You could write it down in long and heartfelt letters that are never mailed. You'd be surprised how getting all your anger down in one place will make you feel better and defuse it.
7- Use negative and positive reinforcement. This is most effective if they don't recognize you're doing it, so be subtle. Not coming over as often or for as long, changing the subject, excusing yourself, leaving the room, looking away, coughing, attending to a child or small animal or anything but them, can be negative. Smiling, looking directly into someone's eyes, patting them on the hand or arm, hugging, etc. are positive (duh). Always be sure to give more positives, but only when the family is doing what you want them to do.
8. Just blow up once. Then apologize. Ahhhhhh, that feels good.
There are probably other ways to help. I've used all of these to good effect. Having family problems eat at you doesn't do your health any good. It's stress-producing and life is too short for the hassles.