Like you, in college I struggled with the lack of structure and nearly dropped out. Then, when I started waiting tables, I began to think I was incredibly dumb, even though I'd always been intelligent on paper, tests, etc... But as a waitress, I was expected to multitask, and fix my mistakes instantly. There was no time to think.
It sounds to me like your problem may be less of an intelligence thing, and more of a multitasking/ lack of structure problem.
I also think you have a different learning style from your sister. Maybe you have a learning disability. It's not something to be ashamed of, especially if you never got help for it. You're writing is clear and concise, so clearly you're not stupid.
Retail may not be the right thing for you. Have you thought of trying office work? Sitting down at a desk might give you more downtime to learn the requirements of the job. My brother has always had trouble following vocal instructions and multitasking (both of which are required skills in retail & service jobs) but he just got an office job, and he's doing great! He's very intelligent on paper, but if you asked him to do five different things in a row, he'd probably seem pretty stupid.
Stop comparing yourself to your sister. You and she learn differently. You might need more structure (like in an office).
Try writing down your natural and learned skills. Then try to match those skills to jobs. This will help you find a better suited job. In the meantime, if you must keep the retail job, ask questions, take your time, put on your customer service smile , and take lots of breaks to think (bathroom & smoking).