Is my sister's beauty in her facial structure or style?

I think you and your sister are both quite similar, you have the same sort of chin as her, and I can tell that you're sisters. You definitely aren't ugly, and you have a nice clothing style. Just because your sister got signed by a modelling agency doesn't mean she's a supermodel or anything; a girl I know has been bragging that she's a model but she's never done any modelling work, and I don't think she's prettier than anyone else. I like how your sister accessorises her outfits; maybe you could try incorporating some jewellery or scarves into what you wear, or you could try new hairstyles like braids, waves or straightening your hair. Also you could try some eyeshadow and mascara, although I like how you have a natural look. You really don't need to worry too much though, I think you and your sister are both pretty because you both have your own style.