Thanks for writing in with your question. I’m glad that you are concerned about these issues and are taking the initiative to do something about it. Your sister’s treatment of you may very well have contributed to the problem, but part of becoming an adult is figuring out who you want to be and taking responsibility for yourself.
Social skills can be learned and improved, but just like any other skill or talent, it takes practice, practice, practice. So in order to feel more comfortable around people, you need to force yourself to interact with others regularly. Don’t just sit in the lunchroom looking at your phone, find someone you know (or someone who you think is kind) and ask to sit with them. It also helps to put yourself in different types of situations — so you would benefit from taking a class outside of your school, such as karate or yoga. Volunteering in your community is another great way to be around like minded people and improve self-esteem, while helping the cause. If you reach out often enough, eventually others will reach back.
Finally, I would suggest seeing a therapist or looking for a support group or social skills group. These are safe ways to improve (and practice) your interactions with others.
All the best,
Dr. Holly Counts