Byul is not aware since no one is clearing things up to him. He's gullible and naive like his mom and we all know what is Hyesung's personality is when it comes to family — he's caring and affectionate as long as it was reciprocated. Now apply it to Byul who doesn't know what his grandma did towards his mom. He might assume it was just a discord between them but there's a deeper reason why his mom doesn't wanna acknowledge his grandma anymore. For sure, his grandma doesn't want Byul to know either about what happened between her and Hyesung as her way to still connect to her "last family that will not abandon her because of what she did before".
And Dojin still in contact with her might be a thank you to Hyesung taking the courage to connect him to the Park Family when he was rebellious before and also, family really do comes first even if we admit it was highly dysfunctional towards Hye's case.
Note: I am not eliciting a fight but I wanna see it in Byul's perspective given that he really do share the same trait from his mother, Hyesung. Brash on the outside but affectionate to the bone.
That was such a well written comment
hey man... maybe if your mom doesn't talk to your grandmother anymore its for a good reason and maybe you know just a suggestion you should respect that