Dear Prudence is Slate’s advice column. Submit questions here. (It’s anonymous!)

Dear Prudence,

My sister, “Sally,” and I have always been close, which means I’ve had a front row seat to her formerly chaotic dating life. There was an era all of Sally’s friends called “the soap opera year” because her romantic life was unbelievable. I was so happy when she finally got into a stable, loving relationship, although I was surprised that it wasn’t a monogamous one. It’s a relationship between two guys who are also with each other. It’s unusual, but they’ve all seemed very happy for five years. Everyone treats each other with love and thoughtfulness. Last year, they bought a house, and this summer, they’re planning legal documents and a traditional but symbolic marriage ceremony. Our

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