I'm going to mention my sisters scar
Why do you want to mention your sister's scar?
Not just that scar, all the ugly things about her
You don't have to like the look of her scar. But she didn't choose to have it and you should keep this opinion to yourself and not hurt your sister further.
She's ugly and she is loud...
Stop seeing the negative side of people and try to see the positive. You can criticize others in private and tell them what you think is wrong, but be kind and gentle with their feelings.