Why am I so jealous of her?

I am not conceited, but I know that I am a good-looking girl, and that I am prettier then most of the girls in my county. But, today my boyfriend and I drove over to visit his mom for a few minutes (he lives with his dad and visits her sometimes). I have saw his step sister in pictures on Facebook and thought she was kind of pretty, but when I saw her in person, I instantly felt jealous because she was drop dead gorgeous. (no homo) Why do I feel so jealous of her??