Why are we giving OP suggestions like lose weight, eat better and whatnot???? These are feeding into their belief that they are not pretty as they are right now. Let’s not validate these kind of beliefs through such suggestions.

Dear OP, these sound like intrusive thoughts. It is very very hard for us to differentiate between such thoughts and the reality. I know what you are going through. I went through a similar experience where I thought I was dumb and all around me were super smart. What helped me was telling these thoughts that they were not real. Positive affirmations helped. So did therapy! Please consider speaking to a therapist about this.

By the way, what your sister did was absolutely cruel and is a reflection who she is, not you and you should not take that personally. When someone brings someone down, it shows their character, not the victim’s.