The only advice I would give OP is that you have to remember, although you all lived through the same things, you will all have experienced it very differently. That’s the nature of life, and how you feel it was as an adult is without a shadow of a doubt very different to how it felt to her as a child. It’s almost pointless for you to try and argue that it was a certain way, it doesn’t matter how you experienced it, she is telling you how SHE experienced it. I know you’ve said she isn’t keen on counselling for the two of you but I honestly do think even if not counselling, the two of you sitting down together, no other kids there and no distractions, and just having a complete and total honest conversation about the whole thing would be really helpful. And I don’t mean you approaching it as you have this post, by telling people how good her childhood was and how wrong she is, or how much worse other kids have it. Just listen.
Firstly, I think it’s always going to be a bit hard when you are the eldest by 5 years and have 2 younger siblings who are very close in age. My cousin is in that exact position and always felt they had a bond she was never fully involved in, it is pretty normal because naturally being close in age they played with the same toys, had the same interests, could do the same activities etc, the older one is always “on the outskirts”. It also means that while your oldest has presumably had to compromise on holidays and trips for them, they obviously don’t have to do that. Yes, it is pretty normal and part of life, but it’s still shit for the oldest. She’s going to feel left out and a bit put out that while when she was 15 she was stuck trailing round legoland, now they are 15 they get to do what they want. It’s a natural reaction for your oldest, you can’t argue with it, just acknowledge that yeah that was probably hard for her.
She’s also well within her rights to be a bit annoyed if you are now more relaxed with the younger ones where you were strict with her. It feels like favouritism to her, and that’s exactly what it looks like. You can’t argue with it, acknowledge that yeah, you didn’t mean to, but you can see why that feels unfair and makes her angry. It is pretty normal, my parents actually did the same with me and my sister, I’m the eldest and it was like being in jail sometimes growing up with lights out times, no tv time, if I left the house had to agree a time back and stick to it etc, now my younger sister is the same age and does what she wants🤣 it is irritating! Acknowledge that!
Being the oldest is hard sometimes, especially if that does mean she has had to listen to conversations of “we can’t do X because of money” and yet you chose to have more kids. That’s how she see’s it and it’s also how lots of adults see it, having more children at the cost of the lifestyle of your existing children does feel unfair. She’s allowed to feel that way.
You admit there was a horrendous 2 years due to PND, so she’s probably not wrong that it was a toxic environment during that time. Of course that’s not your fault, but as a child surely you can see why it has caused these feelings for her? She DID live through that, and you perhaps have no grasp on how she experienced those times. When she tells you it was toxic and stressful etc, don’t try and defend or argue back, acknowledge that yeah, it was probably shit for her.
You’re spending too much time trying to change her mind about her own lived experiences and not enough time just apologising and understanding.