It has been going on for a while and we have not been able to suss out the what or the why for why this family member does this.
The strange, or maybe not strange, thing is that the items stolen are not kept. the person will either dispose of them in the rubbish or wrap it in toilet paper and flush it away. So we can't really attribute acquisitiveness as a reason.
One possible answer we may have got today when asking about their feelings when they did this, was their being annoyed with the person they stole from. maybe it is this simple. maybe not. either way it needs to stop and we don't know how.