You have been given a ton of great advice here. I just want to throw in my two cents:

I was always overweight. My mom is living with me now, so I'm learning a few things about what happened in my childhood. My mom said it was always just easier to shut me up with food, whenever I was bored, angry, frustrated, or had hurt myself. Then, at the tender age of 8, she took me to her TOPS meetings. Wow.

I admit I am obsessed about my daughter's nutrition. But I don't let her see my obsession. I don't want her being obsessed. I encourage her to eat if she is hungry, and I give her decent food choices (she's only 2). I don't play into the shut me up game like my mother did. It's better for her to work out her feelings than to smother them in junk food.

Show your sister love and support for who she is. That will mean more to her than you interfering in her diet. (of course, what she doesn't know won't hurt - keep encouraging your mom to make better choices) I resented my family for making me feel worthless because I was overweight. My mom needed to know everything I put in my mouth - so I hid it. No kid should feel ashamed of eating. EVER! She's growing still, and still working out who she is and her self worth. Don't let it hinge on everyone else's opinion of what is acceptable.

I probably didn't make any sense, and I apologize if it seemed like a rant. I just feel so badly for your sister's situation. Show her unconditional love, no matter her size. She'll have a healthier relationship with food because of it.

PS - I was also 180 lbs when I was 12, and I think back to: if I had been able to maintain that weight (instead of trying to lose it and therefore obsessing over it) I would have grown into some of it, and I would have had a better start when I was old enough to care about it on my own.

I see the weight problem as a side effect of her emotional problems. I think she needs to seek counselling. Sounds like she has a lot on her plate for an 11 year old.

I wanted to thank everyone for your advice in this thread and give you an update on my sister. She's lost around 30 lbs and looks and feels great. My mother and I have both committed to losing weight and I think we rubbed off on her. Plus, she's almost 13 now and really into boys so I think that's helped motivate her some too. Lol.

That is great news !

That's great to hear :)