OP you've just got custody sorted and it sounds like you want to rush in and fix problems that have taken years to build up. This is understandable, but take a bit of time and think about a long-term approach.
What do you want for her at 16? I'm guessing you want her to be a normal weight, but I'm guessing you also want her to have a healthy attitude to food.
I had a friend in primary school who was a bit overweight at that age, probably similar to your daughter. Her parents put her on a diet, tried to get her to stop snacking, etc etc. It led to her hiding food, associating the withdrawal of food with affection, hoarding food, and she was properly clinically obese by the time we left secondary. Girls are exposed to a really properly disordered culture around food and diet.
At the moment, hopefully, the issue is just that she's being overindulged with sweets and snacks and has an unhealthy diet and lifestyle. This is 100% NOT HER FAULT, which you need to remember - she's a child, her diet is in the hands of others, all children would eat junk if given it. Hopefully she doesn't have a bad self-image, or think of herself as 'fat'.
So: the issue is, she's currently overweight and she's surrounded by people passing on an attitude to food and diet that will lead to her staying overweight. Well, all advice about overweight children is generally you don't try and get them to lose weight, you try and get them to maintain it so when they grow, it evens out. So don't put her on a diet, think, carefully, about what you can do to model a healthy alternative and a culture of exercise and self-care. There's no point in you saying 'no snacks' if she then goes home and has snacks the other 50% of the time. If you say 'lets have a banana, not a packet of crisps' she might actually get in the habit of wanting a banana. If you gradually reduce the size of the snacks, then go and do a fun activity at snack time, she might be distracted enough to not miss it. You can start habits of exercise that she enjoys, and I'd really encourage doing it now, before teenage years start.
Most importantly: almost all the women I know with issues around food have emotional eating problems. I think that should be your focus: give her a healthy respect for her body, introduce her to what it can do - cycling, dance, climbing something you can do together maybe - and then encourage her to care for it. You can influence diet while she's with you, but not while she isn't, and I think your focus has to be on building what you want for the future, not short sightedly trying to 'fix' whats wrong now in a way that may cause future problems.