It's wrong to interfere without permission in family members relationship.
Mary just learned that her sister's husband is having an affair with a coworker, but her sister doesn't know about it.
Mary wants to help her sister be happy.
Mary asks her sister how her marriage is going and tells her she's always there for her.
Mary tells her sister that her husband is having an affair.
Mary's sister is happy to feel some support and asks Mary for advice, which she finds helpful and makes her feel stronger.
Mary's sister is completely devastated by the news and doesn't know what she should do now.