We clearly know mc’s “sister” isn’t actually her father’s daughter since there was that fortune teller (?) in the beginning that was telling the father he only had one daughter, not two.
I guess mc’s “sister” is either a scammer, a rare spirit user outside the family, or is from a branch family from mc’s family.
I’m curious if a spirit user can force spirits to NOT form a contract with someone ? Or perhaps each person has their destined spirit that will help them awaken and what happened is that the sister that awakened first contracted that spirit as well..