Maddy is a gorgeous girl. I don't know that I have any good advice. The only thing I can think of is that if you catch her "in the act" of having something that is not hers, is to maybe take if from her with a firm "no" and replace it with something that is hers (favorite toy, treat), with positive reinforcement when she accepts that which is hers.

Any other more experienced PTer's out there who might have better advice. Anyone who knows my babies and me know that I'm not an expert on behavior mods.