Teen suddenly shifts from grief to apathy
You should not write off any concern you have about a family member, and I agree that your sister's apathy in the face of repeated stresses is distressing.
She may be seriously depressed, disassociated or worse. And because she is a teenager (and not yet fully emotionally mature), you should not discount or make assumptions about anything.
Please get her in to see a mental health professional right away. Go with her and speak to the therapist together. It will be useful for all of you to have ongoing professional help.
The American Psychological Association hosts a helpful database of therapists; you can locate a therapist according to specialty and location. Check apa.org. Your family doctor can also make a recommendation.

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