I wouldn't say she's the damsel in distress here ... she could just make up her mind and put some boundries between her & the famous guy who made advances that she doesn't reprimanded, reject etc. "Protect your peace" is very true but in her case, so far after a handful of years she seems incapable of letting go of these people which is fair it takes time. But note that her indecisiveness and inability to let go of these people, is what kept her emotional state in a hazzard. Objectively speaking she HAVE a hand in her current stressful situation.
Her vendictive step sister is very yikes. But they both moved out of the house. They could meet because of family stuff but, I pretty sure Sooae can be clever enough to put move on without forgiving. Have family lunch dinner and treat do the courtesy stuff then immediately give her step sister the cold shoulder. There are ways to keep your distance. She could also tell her parents she has a fallout with the stepsis without being specific, if they don't believe and allow her to keep her distances, that should be a red flag enough for her to not go to these lunches for their approval's sake.
+ If she dislike her current situation she could have just set boundaries, ignore, play dead or dumb and kept her an CEs which she doesn't so it boils down to her welcoming these problematic people's shenanigans and the consequences then play the hurt part when stuff aren't pleasant to her anymore.... Ngl from an outsider view and considering Sooae's personality and social skills, now to and back in high school. She seems like one of those types that has such a small social circle and low self esteem they cling to it and the people that made her feel validated.
sooae's life must be really exhausting. having an ex who ghosed you act like nothing happened and so annoyingly patronizing towards her. a vindictive "step sister" who betrayed her but still have to see bc of their parents. and a famous friend pining on her which makes everything complicated.
honestly, at this point, sooae should just move somewhere else and start over. i know she doesn't have to uproot her life because of messy and fucked up people but what else can she do? all this mulling over is not worth ruining her peace. and that also applies to all of us. if people in our life do not respect our peace, then we should find it somewhere else.