You can't raise 2 kids the same, as overthinking said. I was speaking to an older lady in dhs family, about her own family of origin. Eldest child was very aware of all the financial advantages the family had by the time the youngest child was born, which they hadn't had a decade earlier. She said 'what eldest sibling is forgetting is that they knew our dad when he was still well'. No one could make that happen for the younger siblings.
And for closer together siblings, the fucking joy of being compared to your sibling can lead you to raise your game, or opt out. Kids know when their sibling 2 years younger can eg ride their bike without stabilisers, master grade 6 instrument, always get the countdown conundrum, even if parents don't compare, kids know.
I think birth order plays a huge part in nurture, although of course whether you get decent parents is bigger. You can get people brought up in awful circumstances who are horrible, and others who are fantastic. I think all the fantastic ones must have seen something at some point that taught them how to be kind.
However, with a few bits of extreme medical science aside, once the child had been conceived, you've done the choosing their dna bit. The amount of energy people put into choosing a reproductive partner varies greatly. But once they're here, that's done, and you get on with the business of raising them.