I caught my sister having sex and the image is haunting me

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Dear Aunty Amebo,
The other day I came home after a romantic meal with my boyfriend and walked through the door to find my sister on the sofa with her boyfriend.
They were both naked and clearly having sex. It was disgusting.
I really lost my temper and my sister started crying.
They both gathered up their things and went back to her place. I was so embarrassed because my boyfriend saw them as well.
I now feel I’m unable to leave them alone in my house.
At the moment I can’t even sit on my own sofa without thinking of what they were doing that night.
I entertain lots at my house but I don’t want to invite anyone over to sit on a sofa my sister and her boyfriend have been having sex on.
It’s now even started to affect my own love life as every time I get passionate with my boyfriend that image pops into my head.
My boyfriend just doesn’t understand.
He thinks it’s a big joke, but I’m afraid he’s thinking about her naked body instead of me, as she’s always been the beautiful one in the family.
I want to mend my relationship with my sister because we’ve always been close, but I don’t know how to broach the subject without losing my temper.
  Please help....

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