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Oh for Pete's sake, can we can the Freud crap already? There's enough human drama going on in the LW's family without dragging in the theories of a misogynistic old gasbag. Is there a creditable, practicing therapist who even uses Freud's theories as a basis any more?
Ahem.
LW, it's not your job to tell the rest of your family. It is, however, perfectly within your purview to tell your sister that the details of her affair make you uncomfortable, and you won't listen to them anymore. And then (most importantly), don't listen to them anymore.
It's completely all right for you to express your concern for her wellbeing--dating a string of losers and ultimately unavailable men would indicate that there's something going on for which she may need professional help.
"He just spent a hundred and twenty dollars on a new night gown for his wife. I don't think he's ever going to leave her." "No one thinks he's ever going to leave her." "You're right, you're right, I know you're right."