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@sarahsarahc if she were to kill herself, she wouldn't become immortal because she wouldn't be alive to be granted immortality (plssss this sounds dumb but idk how else to explain it)
the whole bloodline thing isn't something that's passed down, it's just when the most recent family member is killed, the killer is granted immortality. it's not really IN their blood, it's just a benefit of killing them. y/n could kill herself or die naturally, so no one would become immortal. this would then make f/n the most recent ALIVE l/n, so he would have to be killed in order for someone to become immortal. there's also the circumstance where f/n could have another child (which he won't). that would make y/n's sibling the most recent l/n, and therefore she wouldn't have to worry about being murdered, because the target is taken off of her.
i really hope that makes sense and answers your question