I gotchu
He was trying to get Yullur to admit to his feelings so him and Saya could be together.
The series starts off with the twins talking about their mates and how they love them.
Apparently only a small percentage of people could tell it was a ploy.
Yes, I still have that in my head, so you can overlook the fact that he pretended to be in love with Saya. As long as he really loves his mate and not saya. Although I also think it's just a manga, so it wouldn't even be wild if his brother would love saya.
I know, I know, my comment will certainly not be liked but I like the manga. The age difference never bothered me, although it is strange that the brother(even if he's just pretending) says that he loves Saya. If it's really just pretending, then I can overlook it.