Oh OP it gets better as many have said it really does.
When ours were 4 (and a bit) and 2, they did used to squabble but they did love each other, once the 2 year old can join in the crime (of attempting to find hidden biscuits) or kick a ball or get teddy to go to the picnic their relationship will (should) will blossom and watching that is wonderful.
We have two huge teens now and they are close, they have loved the time on holidays together because they have each other, and they remember times and events and giggle away. They grew up practising jokes, memes, silly dances, homework, reading, computer games together.
Board games became much more fun because the pampered eldest learnt to lose gallantly which they totally needed to, and they do learn how to resolve conflict together.
And when they have fun gossip from school they are the first people each other tell.
Providing there is no unexpected disaster, it's a comfort to know they will have each other when we are gone, or too old to be of any great use to them.
We nurtured their relationship above housework, chores, a lot of things really. We decided we would try and cement their bond. And they are different in so many ways (which is great because there is no direct competition if you understand) but have similar outlooks, humour, conversational style.
And they can show vulnerability with each other that I presume their teenage embarrassment might prevent them doing so elsewhere.
It's a long game OP.
"BusyTown" board game used to be a hit here when they were young young.
Potato printing.
Sticker-drawings, so stick some stuff down and draw lines to represent roads, trees around the creations.
And singing, with tambourines (not the most pleasant on the ears) but they it worked.
Failing all this if they were really really cranky chucking them in the bath/paddling pool/even just a toy bubble machine (everyone loves a bubble) but getting them in water seemed to change their mood, so on really cracky days they would have a bath before dinnertime.
It is all worth it. Honestly.