Your younger sister's first relationship.. how do you find it?

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Hi, I'm the oldest brother among my siblings. Since we're young, we've make a promise to ourselves not to get into relationships until we're stable enough to do so. But just recently, my younger sister has broken its long-kept promise. At the age of 17, she had a secret relationship with her schoolmate. She would come home two hours late and texts a lot on her mobile phone. We did not noticed it until my mother and her had a confrontation about staying late outdoors. Since then, she even did not formally introduced her boyfriend at us. When we're asking about their current status, she even can't say a solid statement about it. I feel betrayed and annoyed whenever I remember it. I partly can't accept it because she's just 17 and I feel that she even take her first relationship easily. How can I position to this?