You're insecure in how your family views you.

If your family isn't busy pitting you and your sibling against one another- you're already doing it yourself. Every decision you make is based on how much worse you're doing than your sibling. Don't bother maintaining any ambition, or setting any goals for yourself when you can just leave it up to your parents' successful offspring. Look at it this way- during the holidays and at family gatherings you're unbothered and can eat all you want since you have no appeal and aren't impressive to your relatives!