I understand it's confusing, but it's more than black and white in this story. They didn't love each other in a romantic way, it was more of a guilt induced situation.
Seiji felt guilty because due to his mistake he made Aiji his mate, but breaking the bond will imply hurting his brother and leave him. He didn't want that and the only solution he ame up with was taking responsibility taking his role as mate/partner/Alpha. Remember they were young, without anyone or support, and
I understand it's confusing, but it's more than black and white in this story. They didn't love each other in a romantic way, it was more of a guilt/responsibility induced situation.
Seiji felt guilty because due to his mistake he bit and made Aiji his mate, but breaking the bond would imply hurting his brother and leave him. He didn't want that and the only solution he, a kid who was only seen send pointed for his second gender, came up with was taking responsibility taking his role as mate/partner/Alpha. Remember they were too young, without anyone or support, or someone who taught them well. Aiji knew it was wrong, but didn't want to be left alone so he clung onto him, that's why he said "dragged you down for 10 years"
At the end, they're still brothers, so is obvious they love each other but it was made clear it was not as a lover but as his brother and you can confirm that at the final pages when Aiji asks Seiji why doesn't he bites Towa already.
This plot makes zero sense to me, and i don't understand why the people are saying the twins have brotherly love between them.. what part of having sex and saying i love you in the end makes it a brotherly love? In fact the omega twin was actually jealous that his alpha brother was bonding with a different omega... And they obviously had other options like the omega choosing a beta for his heats because he can't be with another alpha cuz of the bite mark but he can at least be with a beta.. and for the alpha twin he can easily find another omega.. but no, the two of them made a huge mess out of their lives.. another thing i noticed is, the twins relationship was always suspicious, they were already lip kissing or hugging each other since they were little, yeah now some might say its completely normal between kids but the way they were blushing after the kiss was super sus.. they are both messed up in the head