It is very random and just comes down to the set of genes they inherited that determines a lot of their personality. I agree with others who have said that the older of two children sets the tone in a family of two. If that kid bonds well with their younger sibling, the younger sibling will reciprocate. Also I think two children who don't have any other siblings bond better than pairs in larger sibling sets. When it's just the two of you, who else has your back?

Yes and no. #1 and #2 are great friends.#2 tortures #3. #1 and #3 are pals and #1,2 and 3 love #4. #2 is the problem child.

I’ve found the best way to have my kids be friends with each other is to openly show them all how much I love their siblings. Kids love their mom and want her to be happy. If the new baby or the kid with medical issues is causing mom stress, then kids won’t like that sibling.
I think that one of the biggest mistakes parents make is to minimize the amount of affection they have for their kids in case it makes the sibling jealous, when really it’s the opposite.
Kids have a natural affection toward what their mom is affectionate toward.

Three kids, 13 yo boy, 11 yo girl, 8 yo girl.

The oldest two were so close as little kids. As preschoolers they were such buddies that we worried #3 would always be left out. Then once #3 was born, #2 & 3 became super close buddies. They are both girls and do the same sport, so that was a big bonding thing for them.

They all three co-exist happily on vacation together but we were all so busy I don't think they ever really hung out in a friend-like way. However, a friend gave us Monopoly Deal a few weeks ago, and it's such a fun and fast game to play that the last few weeks we will play a game after dinner or before breakfast on weekends. It seems silly but I think it has really bought everyone closer!

Two boys, 10 and 12. They are very close and rarely spend time without the other when they are at home. They have very similar interests so they like to do the same activities. They fight but more playful fighting (they wrestle around like kittens) and they'll chase each other around the house. It's loud and annoying but at least they're having fun.

Twin girls, best of friends and also worst of enemies.

20 year old son and 17 year old daughter are very much friends. They've always been very close and have a friendly rivalry with each other. They have their own friends which they hang out with but still have a day each week to hang out and have fun together.

My kids may be outliers but they just clicked immediately and it seemed to stay that way.