I think what most people don't realise is that we stutterers are already as harsh and critical on ourselves on why we aren't pronouncing words or speaking up for ourselves fluently, so we don't need that extra added insults or criticisms from outside people, especially those who don't know what it's like to live with a stutter. You did not ask to be born with a stutter. Your sister is only knocking your confidence, as a fear of talking and wanting to avoid talking makes it so much worse and less encouraging. I would cut off or avoid people who don't make you feel good whatsoever about your speech, and perhaps look into someone much more empathetic and compassionate such as a professional speech therapist or mature friends to practice with.