@petermushk You're going to give this girl an eating disorder. Food should NEVER be part of a restriction OR a reward. Ever. Food is sustenance and thats it. You eat when your body tells you that you are hungry and stop when it tells you that you are full. Period. Anything else and it stops being about food and starts being about control. Her having to hide and sneak candy is already showing her trying to have control over her situation by eating when you say she isnt allowed to. You are already seeing signs that the stepping stones towards an eating disorder are being walked.
If there are foods you dont want her eating you dont buy them. Stop allowing junk food in the home and instead buy healthy snacks and treats(of which there are MANY) that its okay for her to fill up on. But please, if you care about this girl or her mother(which it sounds like you do) stop having food be part of the situation.
As far as her behavior goes, I think what her counselor is trying to say in regards to giving her a reward at the end of her punishment. Is that punishments need to have a clear set end to them so she doesnt feel as if she is always in trouble and so she knows if she follows through with her punishment she will have all the same rights and permissions youd give to her sister once it is over.
When children feel as if they are the "bad" child they will act accordingly. Especially in a situation like the one you and your gf are in where you are a new element being added to the family and a new person taking her mothers attention away from just her and her sister. Make sure you give your gf and her kids time alone so they still feel like she is as available to them as she was before. Maybe one night a week, regardless of punishments, your gf has a special dinner with just her girls. And makes individual time for each of them as well. Its important for young girls to have that connection and to know that regardless of their behaviour they still get time to be with their mom and have her love and affection.
I've both been the bad teenage girl, and have children of my own(including an almost pre-teen daughter). And I know how it feels to have all the changes going on internally and mentally that a girl her age does. I also suspect I am neurodivergent and I have suffered from mental health and self harm behaviour as well as suicidal ideation in my teen and early adult years. And I have lost loved ones to suicide.
I cannot stress enough that even if she doesnt mean to kill herself, you have to take it seriously and give her the reaponse you would give to her if it was serious anyway. Because you and your gf will NEVER forgive yourselves or forget it if heaven forbid one time you don't take it seriously and she pushes it too far. Kids dont understand the lasting permanence of their actions at this age. You have to be the ones who understand how serious it COULD BE and act accordingly(which it sounds like you are so far).
Dont give up on her. She will know the moment you do. You wont be able to hide that from her. I know its really difficult being a parent and also coming into a situation where you are being asked by your gf to fill a parent-like role in these girls lives. I myself am married to someone who is not the biological parent to my children. And we had to work hard to figure out the right balance and timeframe for my kids as well as my husband for when he started behaving like a father figure with them. We have been married for almost 5 years and he only in the last year started doing more "parental" things like reminding about cleaning up after themselves, or doing their chores, or finishing their homework. So maybe some of the pushback you are getting from your gfs daughter on punishments is because you jumped into that role a little too quickly.
It sounds like from your post and how stressed and out of your depth you feel that maybe its too soon for you, too. Perhaps taking a step back and letting mom be the enforcer of rules and punishments and you just being there behind closed doors to support mom would be better for now? Thats for you and your gf to decide and discuss between yourselves and maybe your gfs daughters therapist. But its something to consider because you having to play step dad too fast too soon will absolutely impact your relationship inevitably, even if you dont see it happening yet.