Me and my sister don't get on at all, and there's absolutely nothing our parents could have done differently. We're just very different people.
I was a quiet "stay in" kind of person, happy to read a book, just a few special very close friends. Sister was an outdoors-type, would talk to/play with anyone. I took my school work seriously and got decent qualifications. Sister couldn't be bothered with school, homework, etc., and barely got any qualifications.
As we grew older, she had a succession of short term boyfriends, lived with a few, married (and divorced) a couple. I only ever had two boyfriends, the first I dumped after a couple of months as he was a lazy waste of space, I took my time to find the second, with whom I've been with for nearly 40 years now!
I think the main difference is that she lives for the moment and doesn't "do" planning, happy to flit between people as she wants whether they're boyfriends, friends, family or whatever, whereas I am a planner and in it for a "slow burn" and have a few very close friendships, very close relationships with a few relatives, i.e. quality rather than quantity.
Our difference outlooks mean we just aren't compatible and are better off at arm's length. She disagrees with basically everything I do and everything I say, and I'm the same with her. She constantly whinges about how everything is so unfair, her poor job, not having a partner, children who barely go near, but in my opinion, it 99% her own fault because of her chaotic lifestyle. She thinks I live a perfect life, but thinks I've just been lucky and doesn't "get" the hours I spent slogging on studying, or careful selection of a boyfriend, of the hours I spent worrying, planning, researching, etc.
Absolutely nothing that our parents could have done to change any of it.