One skill that’ll help you as someone that stutter is to not care about what other people say about you or your stutter. It’s not easy and it took me forever to understand this. But once you understand this, your mental state will be much better, and you’ll learn to ignore people like your sister. Btw this skill is something that monks use as part of their practice to cultivate inner peace. It’s something that highly successful people use to remain focused on their goals and not care about what their detractors have to say about them.