In my opinion, no one can make someone else want to change the way they are eating or lose weight. She's seen your example and the choice is hers. Hard as it is, I think you need to step back and accept that she is an adult and she will make her own decisions for reasons you may not understand.

However, I should add, her comments about stress make me wonder if she is depressed. If this is a possibility, perhaps her husband can encourage her to get help for that condition. Depression can make people give up on themselves.