Um.... Pretty much ugly truth and pragmatic input..... And yeah, this didn't get me much of anywhere. I guess I would just expect that as a professional who spent her working career doing something that requires years of training and experience she would have better sense than to say anything at all to someone of the same ilk, ESPECIALLY since she asked me to come do it because she could not. Was I harsh in my treatment of her in the situation? Yes, but I don't take it well when someone asks me to do something and then tells me I'm doing it wrong. Prolly a character flaw, or maybe subconsciously, since I don't like to be called on with the assumption that I will always come through as a given, I was looking for an out to never have to do it again. In that case, definitely my bad.

As to your assessment of my relationship and my reference to my SO, those attributes I described are most of my attraction to her. I do not see them as flaws, I just try and be there for her when the nasy world steps in and kicks her in the ass. Everyone should be as sweet as this woman is and she brings great joy to my life. Her sister.....not so much.

@NikonJeb I hear you. My wife comes complete with an assortment of incredibly toxic in-laws and even a toxic (thankfully adult) child so you have my empathy there.

It's usually a mistake to assume that too much thought goes into such things. Too easy to infer things that fit, but could easily be wrong and/or less personal than you'd think. Quite possibly in Sis's mind she was just being helpful and saving you the trouble of over-tightening it even more. Wrong, impertinent, but meant well? I don't know. I could be wrong. I might take one look at her and agree that, yeah, she's a straight-up asshat.

But the fact remains, blowing up seldom helps matters. My step-daughter's last time spent with us involved joining us on an expensive family vacation and her acting out was so extreme, so disrespectful, so gaslighting and manipulative and immature as well as actually cruel to her mother, that after several days of that I blew up at her in circumstances that I can say are equally if not more understandable ... but it was still equally wrong. All it did was give her something to add to her persecution narrative as well as an excuse to have a hyperventilating shit fit in response that, for a day or so, jeopardized the rest of the trip and caused us to seriously consider either sending her home by herself or maybe pulling the plug on the whole thing. My wife doesn't hold it against me, and we managed to salvage the rest of the trip, but it didn't help, and she'd appreciate it if I didn't do that again. I will hopefully not, although, honestly, there won't likely be any more such opportunities, at least none that I will willingly be a part of.

So trust me, I understand that sometimes you just break. I'm not immune to it. I don't judge you. But I've learned that being the adult in the room gives you the only real way forward, otherwise it's just two angry reactive people feeding off each other and there's nothing it can do but escalate, at least until everyone has exhausted themselves. It's no way to live. Not for you, or your partner.

But. I. Don't. Wanna. Be. The. Adult. It's funny..... I've buried three sets of parents now, and theoretically, at 62, I don't have to answer to anyone. My SO and I are not married, we have separate residences that we actually both share and retreat to, and 99 out of 100 times, we are pretty much free agents as to not having to be polite or deferential to by bloodline, marriage, or the like. Unfortunately, she does have her sister, yet, but on no level does she get on me anything like she does her sister. Some days I just want to go burn her house down, with her in it, like when I come home and find my SO in tears because her sister was at her again. And her sister gets downright nasty. Her kids are out of control and screwed up, her husband's a total.....jerk, and she's miserable, so because my SO took her own rough situation and did something about it and changed her entire life, her sister tries at every opportunity to drag her down. Hence my short fuse with her.....I will NOT let her run roughshod over me the way she does my sweetie. But I have to remember that my SO still loves her even though she's an asshole. Sigh..... I hate having to pretend to be a grownup....

One last thing I wanted to touch on.... You mentioned that I wanted her to feel remorse... Not so much that, especially since I know with her taking offense to my reaction negates that for her, but I'd love it if it would occur to her how unbelievably out of line it was to question,then argue with me about something I've been doing my whole life. And on top of calling me to do it for her because she could not. It is more likely that she was helicoptering than actually believing that she knew more than I, but I really don't think it has occurred to her how offensive the whole idea was.

It's kind of funny, because as a mechanic, a truly Neanderthal, testosterone laden vocation/hobby/interest, I often run into people that have no compuction in telling me they know more than I, but it doesn't bother me coming from someone who at least has some idea what they're doing. It's funny..... I work in a little one-horse garage in a rural area and we work on a lot of end-of-life junk that becomes an art and science just to keep on the road. I'm learning more and faster at this point than I ever have.

Maybe I should rethink how I deal with the sister in the future. It'll make my SO's life less hassle.

@NikonJeb My father was a lifelong mechanic, first on autos (1930s- 1940s) and then aircraft (1950s-1970s). If I had been born in his era I probably would have taken up the same trade. As it was I took up software development. Both were technician jobs with the bleeding edge tech of its day. Sounds like you've enjoyed your work as much as I have. In life, that's more than half the battle.