How to deal with sibling rivalry with cousin?

Just like everyone else your uniqueness is in your own mind. What other people think of you is none of your business and if you are making your life choices based on what you imagine others will think about them you are really in for a nasty wake up call when reality hits. Most people, even the ones you know well are not that interested in you or in what you think of your self so it is not possible for your cousin to undermine you in any way. Grow a whole bunch of emotional maturity and get a real life not this fantasy life you appear to have.