So you've had a traumatic upbringing with parents together, that makes you think divorcing is better. I have a traumatic upbringing from parents divorcing - the flip side of the same coin.
The difference is, I don't bat my eyes and wonder "why do people not stay together for their children, surely their stability is the most important thing??" because I appreciate that it is COMPLICATED, and that people are by and large trying to do the best thing for their kids, which will be different in each situation.
Lacking that capacity to imagine different people in the same situation as you having good reasons to make different choices, or even different people in a different situation to you making different choices, to act like that is just INCOMPREHENSIBLE enough to be worth asking the question on Mumsnet, frankly makes you sound a bit simple. which was all I was saying.
And yes, I'm well aware that dysfunction is common. This is why I was actually able to get through school, all the way through secondary school, without losing the plot too much, because all my close friends' families, the ones whose homes I was in and out of, were fucked up in their own ways too (either divorced, or horrible marriages, or just generally immature people failing to adult properly) so I just thought all families were like that. We must have all been drawn to each other by sonar or something.
But then I went to uni and met a wider range of people, and my first uni boyfriend's family were like the fucking Waltons, and everyone in my year went home for the holidays like that was completely normal, and I realised how fucked up everything was and had a total breakdown.
I love my step sister, I love my half brother. I consider them my family. But that doesn't change the fact I grew up with my stepmother (who I also love, and who loves me in her own way now we're all adults) favouring my little brother enormously, still does, because he's her blood child and of COURSE she loves him more. It doesn't mean I wasn't enormously jealous of him as a child because he never had to be without his mum or his dad, he never had to choose. It doesn't mean I wasn't devastated when my mum remarried, because that meant she would never get back together with my dad (even though I didn't ever remember them together and knew fine well they were enormously unsuited and had no intention of ever being in a room together again, much less getting back together - I was a little child, and I wanted my family, logic didn't have a thing to do with it).
As the child who didn't have to live through her parent's toxic relationship in real time, I clearly have a hard time recognising that as a privilege. As the child who got to live with both her parents, however imperfect, you don't recognise that privilege. Your older half sisters may have a different take, or they may not. We're all different after all. Which is, well, entirely my point and why "why don't people just do what I did?" is such a bloody stupid question.