Your sister never had your strength or fire but she did at least appear loyal, over the years. Hiding behind a Fidelius ward would be a strong indicator of something to hide, though. Such as knowledge of Draco's whereabouts, or worse.

I wonder if Harry might have gone to ground in the Manor? It's certainly a more comfortable spot to hole up in than the little cottage outside of Hogsmeade. Shame there's no way to slip in and check. I don't suppose you've any influence over the Malfoy elves?