In that drama, they were both "single," which made the beard possible. That isn't the case here. And the more touchy and affectionate they are, whether innocent or for drama promotion, the less straight he appears. Lol.
I personally believe they are more like family than anything else at this point. But I do find it interesting that they are teased but not as heavily as others and no one has ever insinuated they have or are cheating. To me, that speaks louder than anything.
Also, the sister has always come across as the hired help, despite her being family. Though she is absolutely adorable and the way she teases Poon is the cutest. So sad Granny is always spoiling the mood and positive progression.