I'm not a parent either so take this with a grain of salt, but these seem to be the most common complaints I've read from people with siblings.

Don't blindly accept "he did it/she did it" when one of them messes up. It shows them that, for one, their siblings are fall-persons that can be framed to avoid punishment, and potentially, that one of them is presumed the troublemaker if you're more inclined to blame them in the absense of evidence.

Also don't punish both for something unless you have proof they both did it. That's how you make them resent each other.

Give each of them autonomy from their siblings and allow them to do their own things by themselves. Don't force them to let their sibling tag along when they clearly don't want to, they deserve time to be their own person. Again, this fuels resentment as opposed to making them friends with each other, because forcing siblings to do everything together makes them think they're only half a person in your eyes.

Encourage sharing, but don't force them to share everything they own, especially if they're the ones that worked hard to obtain it. Obviously sharing is important and anything you buy for any of your kids should ideally be shared equally, but don't be the parent who watches their oldest kid save up for something they really want and the instant their younger siblings want it, pry it away from them in a misguided attempt to show the importance of sharing. Again, it makes them feel like half a person. Foster an environment where they feel comfortable sharing their stuff, and they'll do it by themselves. Forcing them to share only makes them see sharing as a burden and not a virtue. And when they do share and the younger sibling breaks it, don't dismiss it as "they don't know any better." Teach the younger sibling to respect other people's stuff that's being shared with them and to take responsibility and apologize when they break it.

Don't turn the older sibling into a full time babysitter for the younger ones. Occasionally having them babysit is fine, but if you're, for example, denying your teenager their social life by making them watch their siblings every single weekend while you go out with your friends, they're not going to like you or their siblings. You're the parent who should be making sacrifices for your kids, that's your responsibility and not something you should be imposing on your oldest kids.

Don't say things like "this is the good one" or "this is the rowdy one" to your friends within your kids' earshot, even in jest. Kids will internalize remarks like that from their parents and you will very likely manifest it just by saying it. Also don't twist one sibling's achievement into "why can't you be like that" for everyone else. The kid who achieved something will feel like nothing they do will get them your attention and their other siblings will resent the person they're being compared to.

Mine fought like banshees when they were young but are so close and friendly as adults. I didn't do anything to make any of it happen as far as I can tell. I never understood why they fought so much, and they are so funny and love each other so much now.

Unsure what my parents did but I love and loved my sister. In our case do I think it is because we understood each other so well and gave each other space. We both were good at compromising "if you pick today then I can pick next time" that kind. We had a lot of deals like that between us.

Be the common enemy.

Give each a knife.

The survivor will not hate his/her sibling anymore.

High fences make good neighbors. Most of the really serious conflict between my sister and I was related to competition over use of shared resources like the family computer or TV. Perceived unequal treatment was another big one.

Tell them you love them lots, tell them they have to love their siblings, that family is important because family will get you and support you when none else will, when they're a little older and fighting really bad, walk past them and tell them something like, "you know you two are best friends right?" Celebrate differences, try not to compare, make value judgements or set expectations based on personal value.

Give them a common enemy.

I think there is a lot of luck, because you can't account for taste or personality. Me and my kids all get on because we share a sense of humour, we are not afraid to take the piss out of each other, and we like playing stupid computer games.

Make them unite in their hatred for you.

By constantly modeling respectful good behavior and correcting them when they are rude or mean to each other.

Love them both for who they are and never compare them.

Don't make them compete for your respect and affection.

What !? Siblings hating each other seems exceedingly rare in my personal experience 😅

Strive for equality of treatment.

Growing up my sister and I didn't get along when we were younger, she wanted to give me back to the hospital when I was born.

Over time we shared some similar interests, she played violin, I played cello. She pursued it to a master's degree, I quit to have less divide attention on school which I also failed at.

My parents always gave us options for what we wanted to try and despite her getting more financially to cover those things, I knew I could have those things as well if I asked but I wanted them on my own.

By our early teens any anger was over and was got along for the rest of her life when she died of brain cancer at 42.

Me and my siblings have pretty much always got on, apart from some normal squabbles when we were kids. They're legit my favourite people to hang out with. I'm not sure exactly what my parents did to make this happen. I guess they treated us equally with love, respect, kindness and an excellent sense of humour.

Team efforts.

When people see one another's skills and can come to have confidence in and rely on each other, that builds bonds. Creative exercises are good ways to achieve this. Co-producing a play or video, painting a room, or making a meal (while not hungry, of course) could be methods that help kids to practice this. We take our kids camping and there are lots of ways for kids to work together and rely on each other. Also, opportunities to exercise independent competence and to do tasks that help the family.

Trauma bonding is a dicier strategy. Could work out. Could end in tears. It all depends how many times you want to have them survive a winter plane crash on a mountainside. By the third time, they'd probably catch on.

A lot of good answers here. Being a good model, correcting behavior, treating siblings with equity.

You cannot control everything though. They will have lived experiences outside of the control of their parent, good and bad friends, teachers, etc. They may be born with different medical conditions. All this is impactful in shaping a person, and how flexible they are.

They’re their own people, it’s just a dice roll.

Sometimes time is enough. I'm the 3rd of 4 with my eldest sibling being 10 years older. The other 3 of us are close in age, so through middle school and early highschool, we were shits to each other. Nothing crazy but we were all teens. Once we were all in high school, we all got along a lot better and that's only got better over the past couple decades.

We were raised to respect people and be generally not shitty. We were all treated equally and nobody was spoiled or favored. I think those are the most important facets plus the friends we hung out with. Shitty friends will bring anyone down, speaking anecdotally. A strong foundation of understanding how to be respectful and what's right and wrong has certainly saved my younger brother and I from going down really bad paths.

Not being a psycho POS will go a long way.

Not a parent, not friends with my adult sibling: I suspect having emotionally mature adults around them would help. Also, don't constantly side with the child with easier needs when there is a conflict between children. (Example: I wanted to not hear my brother's music in my room. Brother wanted to play his music. Brother got his way. I got ear plugs.)

Don't make the older child always do the selfless thing because they're "more mature". They shouldn't have to share everything. (Example: brother got to 'help' blow out the candles on my birthday cake. He spat all over it because he was a toddler.)

Why do you think siblings would hate each other? Giving them the mental and emotional tools to interact kindly and calmly with others will also ensure their reltionship is positive.

I know this is a lot more difficult than it sounds because attention is a finite resource, but giving each of them the attention they need. This does not have to be the same amount for each child but none of them should feel like the other one is getting attention when they need it.

Most adults I know that don't get along with their siblings felt like they were neglected compared to the other sibling(s). Whether that's objectively true is hard to tell but it's worth checking in with your children from time to time.

According to a certain website if you add a "step" to their title, they get along REALLY well.

They need space from each other

not possible

By physically separating them until they're all done with puberty.

As an aside, not all siblings hate each other. Or what feels like hate growing up dissipates quickly when they reach adulthood.