DEAR AMY: Fifty-two years ago, my sister shared a secret with me on the condition that I tell no one. She has five children, two with her first husband and three children with three different men (conceived while she was still married to her first husband). I’m experiencing guilt about holding this secret and feel my adult nieces and nephews have a basic right to know their truth. Her fear of being disowned by her children once they know the truth keeps her quiet. Plus, she sees no reason to upset so many families. Is it her secret to keep from her five adult children? The letters I read in yo…