If your mother doesn't want to enact change, all you're doing by trying to force her into losing weight is making her feel uncomfortable. Telling her you love her, that she's wonderful, that she's beautiful, those things help. "Hey mom, you used to be a model, get back to the gym because you got fat and un/less attractive" (I don't care if you've said these words your not, your mentality on the subject is pretty clear), is only going to make her more upset. You said she doesn't like the share her emotions, so you can't even know the depth of how badly it hurts her to think that her kids think she's fat and ugly. Loving the people you love, actually loving them and encouraging them by telling them you love them, and that they have value, and that they are beautiful no matter what they weigh (I never understood how weight somehow got to be the main/only indication of beauty), will boost her self-esteem and happy people are more active.

Love your mother, unconditionally, as I'm sure your father does.