I can’t recomend this enough to people with self-estem issues or social anxiety issues.
Get your ass to the gym and do come cardio exercises. Increase the oxygen flow to the brain. Not only will you be more self confident but you’ll find yourself with a slightly more witty repartee.
Also don’t lock yourself in your home 24/7 because that is enough to depress ANYBODY. Get out there, mix and mingle with the world.
(your sister not you)