Thank you for writing. The fist step toward making change is recognizing that change is needed. You’ve made that step. Give yourself credit for that.
The next step is taking charge of your anger. Your anger doesn’t “take over”. You decide to throw it at someone. Your sister makes the choice to stop instead of being hurtful. You have the power to make the same choice.
As for why you are so angry: I can’t know based on a letter but I can make a couple of guesses. There’s an expression: “Hurt people hurt people.” People who have been hurt in the past sometimes try to prevent it from happening again by getting even — even before someone really hurts them. It’s kind of like saying “I’m going to get you back before you hurt me because I know you are going to do it”. Another guess is that you feel somehow inferior to many people and the only way you know to feel equal is to have the upper hand. Or maybe it is something else. Those are just guesses.
As you are already finding, being so quick to be angry and hurtful doesn’t invite people to want to be around you.
If you can’t get a hold on your anger by yourself, I do suggest you seek out a counselor for some self-understanding and for some anger management training. It is okay to feel anger when you find yourself in conflict with someone. It’s what you do next that either destroys relationships or brings people closer.
I wish you well.