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She’s your sister. It’s not your responsibility to police her actions. If your friends are being assholes tell them to stop and move the fuck on.
And, for a double whammy, sex is fun and their decision - whether they’re having it or not is none of your business.
Be happy that your sister is happy, be there if she needs you - but otherwise, leave it the fuck alone.
That’s a very fair complaint and you can ask them to take it into the bedroom or away from you if it makes you uncomfortable.