Usually the Omega would be in heat right away, but Hana could still run away and keep a more or less clear head until he reached the place he thought was safe. So I think that his brother's pheromones worked so slow on him was because they are siblings.
Rejection occurs only if the omega has formed pair.. That is not the case here.. if the other sibling is an alpha they can cause heats in their omega sibling... I'm not sure which manga you're referring to.. All that i have read.. (and that is every OMEGAVERSE manga available here) does not have an automatic rejection mechanism among siblings..
Well obviously the part where siblings can or not affect each other is up to the author. I just find it a bit harder to describe a world where siblings can affect each other for her. Because too me it means that she needs to take into account that all parents/society in general should be very much more conscious of those fact and probably have systems to prevent it. But that might just be me... Idk, I just find that the short explanation of his back story is kind of...rushed? I probably don't, know all, but I also do not recall any stories where having siblings seemed to be a problem, so I guessed I assumed it wasn't a thing
I've yet to come across an omegaverse where the partner is a true blood sibling and not adopted.. Authors usually don't cross that line.. That's why it has never been outrightly mentioned what would happen if sibling are alpha and omega.. If there is such a case the omega would be kicked out.. As is in this manga.. Because to people omega are of little value and parents would rather nurture an alpha. You're right though.. It does differ from author to author. So we don't know much about what could happen in this one. ╮(￣▽￣)╭
I am very confused on how and why would the little brother induced the older brother his heat? Isn't that impossible in between siblings? I thought they had an automatic 'rejection' mechanism. Well, at least in other omegaverse it never seemed to be a problem, except if one is adopted.