not pretty enough, not thin enough

So, my boyfriend's brother's fiancé is stunning she's skinny, beautiful, has a great smile and lovely long hair. I am extremely jealous of her. 
To a point where I feel like I'm not good enough for my boyfriend's family, I want to be as beautiful and as skinny as her. 
I know this sounds so stupid but it does get me upset sometimes, more than it should. 
Think I just needed to get it off my chest.