I'd say by the hairstyle the brother likes the middle part style always wearing it with longer bangs...the main character has shorter bangs, blonder hair and while he has also a bit of middle part is not the base of the hairstyle.
But for me its also the eyes. Yigon has super white sad eyes and his psycho brother has brownish Grey and they look unhingedand crazy so for me is the psycho look that gives him away. F*king creep
I do know about the crazy eyes/look but in some angle I can’t make them out since like you said they both have middle parted hair. It doesn’t help that their physique are similar as well
I still can't make out Yigon and his ass of a bro. The art is too similar