Question about secondaries

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I would say go ahead. I fail to see why it would be a problem, as her application is separate from yours and what matters is your perspective on the event.
 
I can't imagine this would be a problem at all. Like you said, the essay was written by you and it was your experience with it.
 
I don't see the contraindication. It sounds like you and your sister share a story, but that doesn't make it the same story per se. Good luck