Siblings saying mean things to/about us is not abuse. Siblings do not have any say over our lives, our parents did. Your mother joining in with what your sister is saying is being emotionally abusive because she is an adult you relied on for all your support, or she is being neglectful to both you and your sister by not stepping in and rebuking your sister and teaching your sister better.

What was to believe? Your sister saying you were overweight when you were is mean but not untrue? What you made of it though is internal to you. Some siblings would have retaliated, some denied, some gone inward; like you and I did, since our parents did not support us emotionally and help us learn good self respect at that time. Whether or not you have friends or believe individuals like or dislike you is between you and those individuals. You decided your sister "knew" about these things so just accepted her word without checking it out with your own senses; talking to the people you wanted to have as friends and seeing if they would be your friend.

Hopefully a therapist can help you soon with these issues!