as long as i remember, isn't Yoon Seo Jung was the only child? YSJ adopted by Do In Beom family because of her mother dead, right?
The reason why i thought that Dr. Yun was Do In Beom sister, maybe she got the surname which is "Yoon" from her mother side, when Do In Beom got his surname from his father. And when In Beom got incoming call from Yoon A Reum, the name that written there was just "Yoon A Reum" not "Dr. Yoon" , so i think that In Beom and A Reum not just have a senior and junior relationship, but more closer than that. That's why i thought she was In Beom sister.
But, we can't make sure about this theory until the next episode airing