Dear Annie: My wife and her sister have always had a close but delicate relationship. They can talk for hours, laugh about old family stories and check in on each other several times a week. But every so often, something shifts, and my sister-in-law becomes sharp, angry and deeply hurtful toward my wife. The source of it almost always goes back to her divorce.

Even though the marriage ended 15 years ago, she still carries enormous bitterness toward her ex-husband and blames him entirely for the breakup. During their marriage, however, we grew close to him as part of the family. More important, he is the father of our nephew, and because our nephew lives with him, we naturally see him when we travel to visit. Sometimes that means sharing a meal, catching up for an hour or sitting together at one of our nephew’s school or family events.