Very sorry to hear your sister treated you like this and made you feel like a nuisance for having a stutter. I went through this same issue with my brother when I was younger (28 now) and what helped was to set boundaries that I enforced if push came to shove. For example, I told him that if he made jokes about my stutter I would immediately get up and leave whatever we were doing. Eventually he got the realization that he was hurting me and he couldn't do that anymore. We don't have a perfect relationship but enforcing your boundaries is how you're going to either get past this or realize who actually cares about you in life. Keep your head up and know that people out here care about you!