Dear Prudence This is unsettling. Photo illustration by Slate. Photo by rizal999/iStock/Getty Images Plus and Jacob Wackerhausen/Getty Images Plus.  Dear Prudence is Slate’s advice column. For this edition, Kate Lindsay ,  host of ICYMI, Slate’s podcast about internet culture,   will be filling in as Prudie.  Submit questions here.   (It’s anonymous!) Dear Prudence, My sister “Evelyn” had a bad breakup with her ex, “Paul,” four years ago, and has never gotten over it. Recently, she learned that Paul is getting married in May. She confided to me that she plans on showing up at the ceremony in a wedding dress to demand that he marry her instead! I didn’t think she was serious until she showed me the dress she plans to wear. My question is: Do I warn Paul of Evelyn’s plan so he and his fiancée can implement protective measures, or is this a case of not my circus, not my monkeys?…