YTA. Think back to when you were 15, how you thought about sex, and how you would feel in this situation. She's probably already super embarrassed that you walked in on her, by involving your mother you've made it so much harder for her. If, as you say, you were concerned about her getting hurt, then you should have sat her down and had an adult conversation to make sure she was ok, in control, and understood what she was doing. By shouting and involving your mother without at least informing her that you would first, you have made it way less likely that she would feel comfortable talking to you about this stuff. I hope that your mother is understanding and does not make your sister feel worse about this. It's important that she feels she can access support if she has any fears or encounters any problems.