I had a horrible problem with my little brother stealing my money. One year I decided to start keeping my savings in my mom’s room. Then when he stole it (and he did steal it) she couldn’t put it off as me losing it. Of course, he was her baby and after a short conversation he had “learned his lesson” and was free to go. Shortly thereafter, I opened my first savings account. Looking back on it, I realize he needed the money worse than I did-- he was busy buying every Nintendo gadget invented. I just blew mine on college tuition. Bah! What was I thinking-- that’s what student loans are for. But I’m not bitter or anything. 
Um, oh yeah but back to my point (yes I have one) it turned out to be just a phase. So hopefully this thing with your sister will be short lived as well.
Darnit! This was supposed to be a reply to a thread, not a new thread. sigh
hangs head in shame