Everyone has some degree(s) of sorrow, self depreciation or feelings of insecurity.
One theory about why people allow themselves to be overweight is so they can protect themselves from the pain of living. A layer of fat can act as a buffer, a force field protective zone.
If you are sensative, bruised & wounded on the inside you need a protective layer around you.
Perhaps, by focusing on your sister, it allows you to not focus on yourself, another buffer of sorts.
We each are special, we all look differently, that was God's plan, individuality.
You can make yourself look different on the outside by dieting and exercising, but, the root of the issue lies somewhere deep inside you, and, that must be addressed.
There are many great self help books out there, go to the library, see what you can find. Knowledge is power. I am not a die hard Dr Phil fan, but, he does have some great books that address this very issue.
Eating disorders are a symptom of something else, just as drug addiction is.
Map out a plan, a life plan, set your goals, take one small step at a time, you have to walk before you can run.
Don't compare yourself to anyone else, be you, be proud of you, you are special!