I'm telling my sister that she looks stupid when she arches her back.
Is that something that will upset her?
Yes, she feels strongly about this issue.
You shouldn't tell your siblings their ugly if all its going to do is upset them. She may lash out and be angry at you causing tension with your relationship. It's best to just let the little things go and keep your mouth shut.
And here I am trying to teach her how to be confident. She needs to work on this.
It's not okay to insult your sibling. If you're trying to teach her confidence, insulting her will have the opposite effect. Do you really want to be mean to her and damage your relationship?