Honestly, if she has been doing this for some time and no one in your family wants to address her sternly, then yea, there needs to be a way to show her that stealing is wrong, when people steal, there are consequences. She is only going to get worse.
In my opinion, in addition to investing in a lock for her door, Venetia should get some authority involved. What if she were to steal something from a non-family member who has no problem turning her in?
Edit: I just read
HandbagAngel's comment, and I'm sorry, but 19 is not young. I'm 21 now and I sure as hell knew between right and wrong when I was 19. This is not behavior that should be swept under the rug. Yea, it's embarrassing for the family, I'm sure, but what of it when she gets worse? Without rehabilitation of some kind, she can only get worse.
PS - I hope that my comments don't come off rude, that was not my intention; I just read this over and it can be construed that way. I am just responding very normally to the post.