@MrsKangSiwoo Hm... I was under the impression that it was only because their previous work together was a history drama. So, I was confused thinking "does this mean they can't do another history drama ever?" lol
He did not, in fact, think this through...
@Shinzo it's because in one they were closer than family and in the other they are family who are mortal enemies. It's hard not to see them as the characters who loved each other unless the acting is very well done. Think of any celebrity couple and then you see them fighting after seeing them in love for so long. It would just land differently.