I get the feeling she's gonna be sorely mistaken if she thinks her half-sister is going to be a pushover.
Even though I suspect it won't come up in the story, I can't imagine that anyone who showed up after the grandfather had died, claiming to be a grand daughter, but wasn't registered in the civil registry, would be recognized as such. At least, not without a really expensive lawyer or outright bribery, depending on how corrupt the government is.