Items Deleted, and Difficulty unlocking

Every time I play my MC world, all my items in my chests, furnaces, and I think, but don't know for sure, my inventory, are GONE. Also, my locked level is unlocked and change. I assume that this has to do with my siblings using the same MC, but different profiles, between my games. Maybe it'll help to know that they don't use the snapshot, and they probably play on different difficulties than my world is set to.