I actually think this is often women on women. You certainly don’t hear as often men saying I fucking hate my father in law. Don’t get me wrong, many on either side are simply awful, but for even more I suspect it’s simply an unacceptance on both sides.
I’d also agree it’s a territorial thing in many circumstances. I think it was with my mil.
My own mil at the beginning was horrible to me. When I was graduating I had been with my husband about six months and stayed at their home, I had food poisoning and was vomiting, and she came into the bedroom when I was alone and angrily demanded if I was pregnant. I was genuinely quite shocked. I wasn’t and was horrified she was thinking that. Id only met her twice.
Another time we were going to an event and got all dressed up, I was early twenties and was wearing a skirt that was short, just a bit lower than mid thigh, I said to her “does this skirt look ok” and she said “you look like you should be standing on a street corner”.
They were solidly middle class and I grew up in a council house and was dating her youngest son, and she made it clear I simply wasn’t good enough.
That is until I had her first grandchild. Ironically I’ve been with my husband now 31 years and I am the main bread winner, although he’s successful in his own right. I ended up being the one called in to nurse her at the very end, and I did it and cared for her, but it was always there for me.
My husband had also had a several year relationship with a local girl, a friends daughter, and the relationship ended because she cheated on him repeatedly, but I think mil wanted them to get married, and even went as far as inviting her to their home once when I was there and we’d only been going out a few months. I expressed I thought that was a bit uncomfortable for all of us and my fil was highly amused and said more uncomfortable for her, he’s clearly not interested, not sure what mil is playing at.
I do think there was an element of territorial, jealousy and being protective, but ultimately it was a woman not accepting another woman.