Like theday said, there are probably siblings that won't get along whatever you do. But that's no reason not to try: there are all sorts of undesirable things that could happen to your children and that do happen to some children; doesn't mean you don't try to avoid them.
I was one of 4 and as a child got on noticeably better with my younger siblings than my eldest, but we are all on excellent terms as adults and have many shared happy memories- I think that counts as a success.
Some things our parents did:
Never showed favouritism. My mother has often said "My favourite child is the one I happen to be thinking of at the moment"- and we believe her!
Give to each according to their needs- there wasn't any anxious reckoning "if you've spent x amount of db's violin lessons you've got to give me the same at once"- but the reason there wasn't was because we trusted them to even it out over time.
Not permit physical fighting. Every single fight broken up with the simple motivation that we have certain standards of behaviour in this household: you're not allowed to fight any more than you're allowed to put your feet up at the dining table. Fights broken up, sent to separate rooms, whatever it took, just not allowed. This avoided the resentment that arises if a physically weaker child is frequently hurt by a stronger one.
Insist on certain standards of speaking to each other- no name-calling, no insults, no goading. In fact, I think they could have been a bit stricter with me on this score as I was a precocious little brat and always trying to outshine my older brother.
Shared memories. We had outings together, picnics, we travelled a bit, we joined in with things like baking and DIY. What my parents got really right imo is that they wouldn't let one sulking child spoil everybody's fun- they ploughed on regardless and showed as little resentment as possible. It took the power away from the sulking one.
We are very different people- two of us academic, two not, different personalities, different paths in life, but those shared memories and skills give us something to talk about.
Also, shared memories can be used for bonding: making disasters into a funny story for the family to share and bring up again. Don't be afraid to tell a story against yourself- it shows confidence, which is not a bad thing. And a silly story handed down through generations reinforces that however different we are, we belong together.
I noticed with my ILs too, how good they were at using shared memories- not least funny stories about when things went wrong- to reinforce bonding between their adult children.
The mistake, I think, is when you make a very conscious effort (and spend lots of money) on Creating Perfect Memories, days when nothing must go wrong, and everybody must be in a good mood, and nobody must get cross or be starting a cold or spill their coffee on their best outfit. The good shared memories, the ones that help to bond, are when you're hiding from the rain under a coat that has to stretch over four people, or when mum sits on the ice-cream, or you miss the bus and have to have an improvised singsong while you wait an hour at the bus stop.
My nieces and nephew are now in their thirties but they still talk fondly of the time when we had to wait for an hour for the boat from the Isle of Wight and dh and I did an improvised fairy-tale session with all the facial expressions and gestures. Over the years, this has settled into a ritual formulaic retelling which ends triumphantly with the last person leaving the bar on the boat as dh falls into the boiling cauldron.
Some stories are so good they end up being handed down through generations. Very often they are the ones about disasters or silliness (greatgreatgranddad helpfully offering to post a parcel for the maid only to discover he had posted off his own silver spoons, grandma cheeking an aunt, dad being carted round the neighbourhood as a Guy by his big brother on 5 November). What they do is reinforce the feeling that however unlike me those people are, we belong together, there is a connection. And that can help with sibling bonding too. The point again is, don't make it tedious stories about how clever and admirable people were. Silly stories work.