Have you thought about saying in front of your family "Someone keeps taking money from my wallet. This needs to stop."
Since I really want to think it's not your mother or father doing it, I sure they would be interested in the fact that one of your brothers or sisters is taking your money.
As far as hiding places, do you have a pair of shoes or boots in your closet you could put the wallet in? I don't think they would think to go looking through all your shoes.
Or, you can do the old carve out a hollow in an old book and put it in there. All you'd need for that is an exacto knife to carve out a bunch of pages, and some mod podge or elmers glue to brush around the outside of the pages so the book forms a kind of box.
After you do that, buy yourself a cheap wallet and put in your jacket pocket. In the wallet put the note below for whoever is taking the money.