Making up lies about someone cheating is immature.
Mary is mad at her sister who took Mary's favorite dress without asking and got it stained with red wine.
Mary wants to react to what her sister has done.
Mary has a serious conversation with her sister, explaining how angry she is with her for ruining her favorite dress.
Mary tells her sister's boyfriend a made-up story about her sister having an affair with her best male friend.
Mary's sister takes responsibility for her actions, apologizes to Mary and buys her a new dress.
Mary's sister has a huge fight with her boyfriend which puts a strain on their relationship.