My mum never bought biscuits, cake, crisps or otherwise for the house. We would get it maybe on a Saturday night if we had a video (old alert) or if we were out somewhere. It wasn't part of our daily diet.
I have never bought any of the above for our house either and I do this without thinking about it. Why would I? If I had biscuits in the house - id eat them, daily. If I ate biscuits daily Id be 2 stone heavier by the end of the year.
If I know people are coming I buy biscuits or on the odd occasion bake something but that is it.
Really think about everything in the house. You really need to re think your shopping if you want to help her which obviously you do.
Nobody NEEDS sugary cereals, biscuits, crisps, chocolate, sweets or anything like it. It isn't a food source so don't buy it.
They will moan at first but start telling them the shit that is in there and what it will do to their growing bodies. Don't mention weight as such.
My son is 6 and knows all about MSG's, GMO's, sweeteners etc. He also knows about Antioxidants (or antioxegens as he calls them), vitamins and minerals.
If you talk to her about how bad food can be for her body but also how powerful and good it can be you are educating her for life.
It is sad that she is overweight already but that is just a small problem. Her health is under major pressure at a time when she needs nutrients more than ever. Puberty is the most demanding time for her body.
I watched my 3 year old inhale runner beans tonight because she wanted to run faster than her brothers
.
I know an 11 yr old won't fall for this but there are other ways to put it to her.
Maybe tell her how all of the stuff in fizzy drinks, biscuits, processed foods etc is linked to cancer and how you really want the family to be much healthier at a time when food is getting worse. Mentioning her weight will just make her feel bad about herself.
Then tell her how nuts and seeds are amazing for beautiful skin and if she sprinkled it on her cereal everyday it would have her glowing and sparkly eyed within weeks. I don't know what she is like but you could find away of talking to her about it so it would be of interest to her if you see what I mean.
Good luck. I hope I have helped and not made you feel worse as I don't mean to. I just find being negative about processed food and positive about healthy food is so good for kids and they really do respond to it.