What your mom did was right. Your sister broke a family rule and your mom was enforcing the house law. It sounds like she was quite decent about it under the circumstances (e.g., knocking first.) I don't think there is any fundamental right to privacy in this situation.
Further, the generalization that "nobody should just walk in..." really doesn't hold up. What if (non-statutory) rape is suspected? What if your sister were 12 years old? I would even go so far as to say that under certain circumstances parents have a moral obligation to "just walk in."
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