Jesus Christ you lot are nuts! Do you think every action anyone does is about a trauma, emotional issue, not being loved enough blah blah...
The kid is taking food without asking and not caring that she's not being fair - she's being selfish and greedy. It is bloody well stealing and i get pissed off with it in my own house.
The other day for example i was making a recipe (a tagine) I had bought dried apricots for it and they were in the cupboard, I went to get them - gone. I was cross, of course i was. Just because food is in the house, doesn't mean it is a free for all for anybody to have at anytime. Especially if you're having to queue for half an hour just to get into a supermarket!
After queuing, spending more on food than normal as everyones at home, (3 teenage boys) if i buy a 6 pack of Penguins or whatever and find them all gone and eaten by only one boy a short time later, then yes that is stealing food and i would be (and have been) fucked off about it.
You lot have ZERO insight or real idea about the OPs situation, her family dynamic or anything at all really. YOU DON'T KNOW HER ffs. So quit the armchair, holier than though, psychology bollocks and look at the issue of her step-daughter nicking food.
OP tell her off! Its discipline that's needed not a fucking counsellor. OMG. 🙄🙄🙄🙄