At 3.17 pm most people are at work. You sound as if you have a bit of a persecution complex! Nearly all IL clashes are down to, as you say yourself, differences in norms between families, and people being thrust into a semi-familial relationship with people they would never choose to have in their lives, rather than anyone being actively malicious.
My DS is only 12, so I’m nowhere near the DIL/SIL stage, but I can tell you exactly what my (very nice, but terminally unimaginative) MIL’s beefs are with all of her three DILs.
She finds her French DIL ‘too grand’, ‘chilly’, and thinks she doesn’t make enough food when she and FIL visit them. I understand what she means, but it’s largely cultural misunderstanding. My SIL is reserved, formal in manner, grew up in a small chateau, is horrified by the vast amounts of processed food and meat served at PILs, and is ready to die from how hot their house is kept when they visit (from overseas, so overnight stays necessary, so she deals with it now by no longer ever visiting, ever, under the guise of work.)
Her second DIL she blames for a longtime rift with her son (who is estranged from the whole family, did a lot of prison time, and seems to have considerable MH issues) — she may have a point, but the initial rift predates him meeting his now-wife, and their wedding actually involved a brief thaw. I get that MIL is very sad about not seeing those grandsons, but it’s a lot more to do with her son’s poor MH than who he married, as far as I can judge. From a distance, as we haven’t seen this BIL and SIL in over 20 years.
I am too career-minded, not house-proud enough, and am a dead loss because I’m not interested in sport. Plus I ‘took DH away’ by studying and working overseas. She also can’t get past me having one child by choice when she had seven. I remain very fond of her, as DH’s mother, but she is terribly tactless and unable to put herself in other people’s shoes. She’s said and done fairly awful things on a number of occasions down the decades.
BUT the reason I am the only one of her DILs who is in regular contact, and makes a point of taking DS to see her when DH is away, is because I am a flexible, imaginative person. I can see that her early life (eldest of 13, very poor, lots of problems) meant that she had to become a battleaxe to survive. She had no opportunities, no education, she was taken out of school at 12 and had three children by her 21st birthday in two rooms over a shop. A sensitive, imaginative person wouldn’t have battled through.
I could list genuinely hurtful things she’s done to me down the years, but none of them were malicious, just thoughtless. I just don’t register as ‘readable’ as a personality for her (similarly poor background, and eldest of a large family, but the age difference meant I was able to get to university on scholarships and escape.) I understand her, she can’t understand me, so I am the one making the effort to keep the show on the road in DH’s absence. Even though I’m not the DIL she would have liked. None of her DILs are.