I feel insecure around my sister?

I don't have a sister but I did have foster sisters and had a similar situation to yours. The best thing to do is find the things you are good at and the things you enjoy and work on excelling at those - not to be better than your sister, but to be better for yourself. Instead of comparing yourself to your sister start finding things to do and get involved with that your sister is not involved with. Here's why...if you do different things than your sister it will be harder for people to compare you to her. But ultimately, this will help you to find yourself and become comfortable in your own skin. It won't happen overnight, but it will happen. Don't focus on losing weight; instead, focus on keeping your body healthy. You sound very smart and very aware of what is going on around you. I think you just need more time to get to know who you really are and to find yourself outside of what your family thinks you should be or what your sister already is. You are your own person and as you work on making yourself unique people will slowly begin to see and accept that. You have to be comfortable with yourself before anyone else will be. And once you are...I can tell you honestly you won't even care if other people don't accept you.