
I didn't read the raws, these are just my speculations..
Maybe the hot brother also goes only for alphas or likes to play around like the doctor.. and Doyun knows that soooo he might be afraid? that the bro and doc might do it or something?
Either that or the hot bro over there did something to Doyun (or maybe his lover from the past) before, so now he's afraid of him?
Can someone who read the raws enlighten me please? (⌒▽⌒)

Actually it is foreshadowed in this chapter here. That brother said some really horrible things to Dojun when they were kids. I think he still has a trauma because of that and his complex might be from that time as well. His older brother is hot, but he seems to be an asshole, so I wouldn't be surprised when it turned out that he was so mean to Dojun all the time, while they grew up. http://www.mangago.zone/read-manga/love_shuttle/uu/ibx_chapter-37/pg-1/
Fuckkk