First problem I think is how you feel undermined by your younger sister. That's a problem because if you truely think that you'll be faced with it everynight at home etc. Your sister may be attractive then by default you must be too ^^ same parents I assume?
NEVER put yourself down, seriously nothing comes from putting yourself down. Just think positively, optimistically etc. Just try to talk to people, you'd be surprised how willing people are to talk. I love talking so when someone starts talking to me I'll automatically listen and develop a sense of respect for them. About being confident in yourself that shouldn't really matter. I'm not confident in myself, but confident in my beliefs that other people will accept me dispite my inhibitions. So it's more have faith in your ideas and thoughts rather than yourself if you understand? In the end confidence is merely a placebo and it's easily gained. Smile at yourself in a mirror, sounds loserish but really it will help make you just feel generally more happy.
So to sum up, talk more
smile at yourself
believe in yourself
don't let other people ideas influence you
Hope I've helped ^^