Hikaru thinks he knows why Kaoru doesn't get jealous.
It's because Kaoru has everything he could ever want. Not that he's spoilt, just simple.
Kaoru just wants his brother, Hikaru knows. And as long as Kaoru has his brother, he won't be jealous of anyone.
Hikaru could go to bed with a million different people and Kaoru wouldn't care, just as long as his brother came back to him. Hikaru could do things that would leave others angry, but Kaoru wouldn't get mad. He'd smile that smile of his and laugh, telling Hikaru that he couldn't possibly get mad at him for anything. And when Hikaru asks—demands—why, Kaoru'll shrug and say that they're brothers, and that sure, sometimes Hikaru pisses him off, but at the end of the day, he still loves him.
And Hikaru'll grit his teeth, and he'll yell, and Kaoru'll just stand there, taking it all in. And when Hikaru storms off to their room, Kaoru'll go to the kitchens and get a box of maple syrup, which he'll then take up to Hikaru. And they'll sit on the bed, taking swigs of the syrup every so often, and then Hikaru will apologise. And Kaoru will smile and laugh, and tell him that there's nothing to apologise for.
And then Hikaru'll smile uneasily, before Kaoru declares a Syrup War and they both end up on the floor, laughing and wrestling, covered in the sticky substance.
And Hikaru knows that Kaoru will always be waiting for him, even when he makes the wrong choices, because they're brothers, and at the end of the day, they love each other.