i don't think you're being silly at all. i think a lot of people with a sibling who is very close to their age probably have similar feelings, and i'm sure it's worse for those who share even their birthday with a sibling. it's really hard to feel like you can have an identity that doesn't in some way involve your sibling if so much in your life is similar to theirs--how could you help but compare yourself?

i think it would be really helpful for you to talk to someone completely unfamiliar with your sister. that person would only know you and, consequently, be able to help you see yourself in a way that is completely independent of your sister.