DEAR ABBY: My sister “Sandy” divorced her husband 15 years ago. Her oldest son, “Nick,” has never gotten over it. He’s 30 now and has treated her badly over the years. He not only vandalized her home but has also said horrible things to her. She maintained a relationship with him despite his behavior, supporting him emotionally and sometimes financially. Sandy has been engaged to her long-term boyfriend for three years, but Nick is hostile toward him. He calls him names, denigrates his faith and refuses to accept him as part of the family. Her fiancé has grown tired of this treatment, and I’m …