It's Two People

[From NetComics]:

Ordinary high school girl Jina discovers that she is heir to a terrible legacy handed down from her family's sinister history. Long ago, her ancestors killed a magical serpent known as an Imugi, believing that it would bring them good luck. Unfortunately, the creature cursed them as it died, decreeing that one family member of each generation for that day forth will be killed by two people closely acquainted with that person. One of Jina's relatives has been murdered without fail in every generation, and now, Jina has been informed that she will be next to die.