Are you saying the age gap is problematic but the sibling thing isn't?
The sister thing IS WIRED, but I don't think it's a situation where the Brother LOVES LOVES the sister.
I think the brother just likes his sister (in a family way) and doesn't want to see his sister with someone else (kind of how fathers are when their daughters start dating). He doesn't want the sister to grow and just wants to hog her all too himself because they spent a lot of time together since they are siblings.
Is the brother wrong? YES
Is what he doing wired? YES
Does the brother have romantic feelings for his sister? NO
Everyone in family is like this. When you have a younger sibling, you hate them because they are hogging mom's attention. Does this mean you love mom in a romantic way? HELLNAH. When siblings get along, they can also get jealous when their sister/brother starts dating.
For a genius he is really dumb. To get jealous over your sister spending time with another man, I mean you are prioritizing your own feelings over logic. It's bound to happen, instead of being jealous and acting on impulse (not a genius thing to do) you can instead adapt to the situation, that's what geniuses do after all. Childhood doesn't last forever, it's time for the brother to be with someone, someone like Miu?