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.

Don’t make them 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.

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.

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.

So my experience is a bit different as I had teen mom with ~5-7 year gaps between each siblings. Personally gaps are real nice if you have a choice. Less competition. But I think the real answer is team activities that are not competitive. I played comp stomp with siblings regularly. Taught my brother to play rts at 3. We still play together 25 years later. My youngest sibling just wants to do their own thing and the older ones didn’t encourage playing together so she’s more of an outsider. I remember she complained about it when she was like 5 but I wasn’t around much then to force it. I told the others to step up but they never did. If they did I wonder of things would be better between all of us.

Be the common enemy.

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.

Give each a knife.

The survivor will not hate his/her sibling anymore.

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

not possible

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

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.

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