I know this sounds bad but is she really gonna see you eating or not,are you gonna be close enough,I was so unwell at my sister's wedding,I had a bug and couldnt eat a bite and nothing I could actually do about it,I was Bridesmaid and was seated at the top table so everyone could see me sitting there like a numpty not eating but it happens.Yes,she shelled out for the meal but at the end of the day,she wants your company on her big day and neither of you will enjoy it if you feel uncomfortable and you both have had a barney.Take it as it comes,doll yourself up and go with the flow!