i dont think so either. in america even if the parents got married, as long as the kids were not raised together most people would just sort of laugh and joke about it being weird, but not really mean it.
now about the mother's illness, i dont even think this adds up to much of anything even in k-drama land. if the woman needed alot of care, i could see that b/c in korea things like sleeping over and living together out of wedlock aren't really respectable so they young lovers wouldnt have time to see each other, b/c she'd be too busy taking care of her ill mother.
now joon's thing with his mother i can kind of see. through my own experiences i know what it's like to want to protect a mother who has been ****ed over for decades. i can see it giving him pause.
all and all though i really didnt like this one. for a melodrama i guess it was good, but it failed to entertain me, with the exception of episodes 5-10.