DEAR MISS MANNERS: What is the appropriate way to refer to my brother’s long-term significant other? “Sister-in-law” wouldn’t be correct, since they are not legally married and will likely never be. “Girlfriend” seems too casual to correctly reflect the depth and seriousness of their relationship (they have been together over a decade, and we hope they will be together forever). “Partner” has often been construed to mean same-sex relationships and leads to awkward assumptions and incorrect pronoun use by others. “My brother’s significant other” makes for an awkward mouthful in the middle of wh…