My sister has always been slim whereas I have always been overweight. She has always worn the sort of clothes I would kill to wear, like gorgeous fitted dresses and strappy tops. I don't feel resentment towards her as she is my sister and I don't mind her looking attractive. The issue only began to effect me about 6 months ago, when an elderly relative stayed with us and commented about 5-6 times over her stay about how beautiful my sister was and how pretty she looked. I found it very hurtful as it came across as very tactless e.g. she was the attractive one and I am the ugly one. So yes, the desire to not look fat and ugly next to my pretty slim sister has kicked in, thanks to that relative!!