Here is your problem:
You don't know how to assert yourself in a healthy way and so until you do, you will ALWAYS be a target of abuse not only for your sister but anyone else (like the customers). So, why do you take it?
In your words you are...."I look very nice and quiet." And so you want to be a target for abuse?
By the time you are 18 and an adult, you should have learned how to assert yourself to others. You do it by remaining cool....do NOT REACT OR ACT OUT when you are being bullied. You must remain calm at all times and keep good eye contact with the person you are dealing with....your sister or the customer.
And the MOST IMPORTANT THING is to TAKE CHARGE OF THE SITUATION YOURSELF!
If you have made a mistake scanning something three times, just remain calm and say so...."Oh, it looks like a mistake here so let me correct that right now." (You are in charge of this situation and you have made it right.) there is no need for your sister.
When she butts in, simply look her in the eye and say...."I think I've got it from here, thanks." and take charge.
When she makes fun of you, you look her directly in the eye and say, "Excuse me. Would you repeat that?" and then stick up for yourself....you don't need her to stand up for YOU....you've got the situation in hand because you are in charge of it.
You need to learn how to be independent, self-reliant and responsible totally for yourself.
Your customers are in all sorts of different moods but remember they have NO POWER OVER YOU....you are in charge of checking them out and you are in control of it....that's what you're paid for.
Time to grow up.
Go to websites that teach healthy ASSERTIVENESS SKILLS.
This is something you desperately need to learn now.