This is tough, and touchy.
My suggestion is not to focus on how much she weighs or needs to lose or is gaining, but to instead make the focus on the food choices. "I'm concerned about how and what I see you eating, Mom..."
Of course, this shouldn't be thrown at her when she's in the middle of eating something, more like you should try to catch her when she's not busy or stressed right at that moment.
"...I love you. I want you to be around for a long, long time. And... the things I see in this house aren't going to lead to that... even my little brother's health is going to suffer if he's eating these things constantly... we don't need to keep them in the house, not good for any of us..."
Make sure you're sitting down with her. Make it about concern, never judgement. And if you bring up the rest of the family eating better then it's not just about her, but everyone in the household.
"...Would it be okay if I come along to help with the shopping? We could go as a family for a nice trip and pick out better things to eat together!..."
Offer help. You can't force her to accept it. But offer it just the same.
Only she can decide to eat better, or to lose weight. Show concern, but if she doesn't want to change, then she won't.
Be careful not to bully her or belittle her. (Not that you're doing it now, but it's easy to take things the wrong way.) It will probably just add to her stress, and cause her to eat worse.
I hope that she's able to see what you're saying, and change for the better.
