Step-Sister sent me nude selfie?

So, just last night I received a nude selfie sent to be by my Step-Sister - why she sent it to me I have no idea and to be honest I didn't want to see it - I was quite upset actually. I take it she sent it to me by mistake since as she hasn't mentioned it since around the House or in the Car. After living with her for 5 years now I'm surprised she actually did this.

Should I tell her or just overlook it and keep it secret? It's shocked me and not in a good way.
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I came back yesterday from College to find her fast sleep in bed in the middle of the afternoon - it was pretty weird she sleeps in the afternoon quite regularly - I can't even do that though I tried.

I travel with her to College everyday like today I was pretty awkward after seeing this picture she sent me.
Step-Sister sent me nude selfie?
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