I would just make a point of following her around, like hugging-distance following, whenever she's on your property. Literally put your nose into whatever she's doing...and just wait for her to comment on your actions.

Then you can say "Well things have been going missing, several times now it has been you, and I'd just like to catch you in the act for once so my brother won't be mad when I bloody up his wife for being a no-good lying theif. That's all!"