What do you think about me calling my younger sister a liar?
Why do you want to call her a liar?
Because she called me a liar.
If you really think that your sister is lying, then it's okay to try to get her to stop. However, if you are just responding to her accusations with accusations of your own, then this will get you nowhere.
It's a habit. Besides, she has to learn. I already told her it's ok to talk.
You shouldn't call your younger sister a liar just because she has said this to you. Instead you should lead by example as the older sister and talk to her about how what she says hurt you.