i think he is one of that typical lonely ML who fell in love with someone who showed him kindness, regardless the gender. (probably) in the original novel uke showed him affection, kindness, etc that he never received his whole life that's why he fell for him, but now since the sister is reincarnated and she's trying to create happy ending he received kindness from her first, before he met uke that's why he fell for her.
i think he is one of that typical lonely ML who fell in love with someone who showed him kindness, regardless the gender. (probably) in the original novel uke showed him affection, kindness, etc that he never received his whole life that's why he fell for him, but now since the sister is reincarnated and she's trying to create happy ending he received kindness from her first, before he met uke that's why he fell for her.