Sohan does really want to kick his father out but when he saw his mother happy he can't accept the fact that while his mother is happy but what about his father? We can see that Sohan felt guilty about it even though it is not his fault because he is his family. Sohan just want to live peacefully without guilts and burden so he just want both his parents to be happy while Sohan is also happy out of both his parent's life.
While for Sihwa, I felt that he would not be that hurt if he knew the truth because I know he is more curious and focused more on Sohan and his an Alien so yeah. ╮( ̄▽ ̄)╭
Yeah but honestly it means you don't trust him enough to tell that but I hope he won't be that insecure but just be worried and nonchalant about it