How does this work?

So, I love my mother-in-law. She is a really sweet person, and so generous. Whenever the husband and I dog sit, she insists on writing us both checks for our "trouble." This past time, she left John and I our own cards with checks inside. We decided to open them together. His says, "To Brother John, thanks, wuff, wuff, etc." and mine says "To Auntie Clarissa, thanks, wuff, wuff, etc." Um. o.O So my husband is my nephew? I had no idea our family tree was more like a bramble.