I think some of it is just time. My two DS are in mid/late 20’s. Not close since eldest left for uni, and they went through usual teens falling out before that. Up to age of 11 they were very close
Once they were at Uni, there were only weekends and long holidays to see each other. And their peer group were their priority not sibling for socialising. Now they both work and live over a hundred miles for. Each other, seeing each other’s means a weekend visit. When you’re young with no kids weekends are in short supply for doing that young, free single bit …they want to go places, experience stuff. They do go stay with each other maybe once or twice a year, but it is fitted in around other things.
I don’t get worried- I was no different at that age with my siblings. I also think lads are worse for staying in touch. I made more effort with both my db , they hardly ever made the effort with each other,
When we all settled down and had kids, that carefree lifestyle sort of went by wayside and we did start to see more of each other, even though , like my sons, we lived a long way away from each other.
Now we’re in out 50s we keep in contact very regularly. Oddly I moved closer to one db geographically and see him at least once every couple of weeks for short bursts.
It isn’t about how often they see each other, that maybe difficult at their age, it is about keeping the bonds going in the background. You can help facilitate that by, for instance, arranging group face time calls . Doesn’t have to be a big deal , just simple chat. I do this a lot now. I organised one the other day as one of them just mov3d to new rental so I set up group call so he could show us all around the house! Just keep fa ilia tasting at this stage to keep them briefed on the bog stuff in their lives. It’ll gradually work itself out as they get older.