You don't need to compare yourself with your sister. Or anyone else. You are two different people. Is your sister so perfect that she is the gold standard of a human being-beautywise or whatever? No, no one is.
I also dissapoint myself regularily because I have a severe problem with prioritizing, self-motivation, procrastination...I screw up-I beat myself up for it for a while-then I try and figure out what went wrong and why-and I try again. and again...and so on....
That's all we can do, is keep trying to accomplish our goals. And learn about ourselves in the process.