We didn't see anything from her previous life and those around her, hence we don't know if she didn't just shun everyone else in her blind rage. To top it off, Yulan seems to be the type of person that would dislike the MC if she truly turned towards bullying her sister, even if it was understandable in some way, I think he would have fallen out of love if she saw how ugly and twisted her nature turned.
I for one am convinced that she didn't even pay attention to Yulan since she was so thirsty for the prince (which would be another issue for Yulan since that could drive him off). But I'm glad he gets his chance now.
I think it's too premature to describe Yulan's character since there is only much we've obtained about him till this point.
I believe it would develop farther on the future chapters.
Vio did say in the first chapter that because she was too hung up on maryjun, she didnt see the people who worry for her
What I don't understand is why in her past life when she bullied Maryjun and tried to kill her Yulan didn't try to stop her or reach out to her? Maybe he did, but from MC's memories this doesn't seem like it. Yulan obviously has always cared a lot for Vio and seems to understand her feelings, so it is very strange he wouldn't try to stop her even if it was just to make her avoid a (obvious) bad end.