I've had this problem myself a couple of times, finding a "cousin" or "niece" in the census, doing contortions to figure out whose sister the head of the household was married to, only to stumble upon something that revealed that they were related to the wife. Sometimes I need to smack myself and remember that enumerators made tonnes of mistakes, and I'm just as prone to hoping for shortcuts. (I also think there's a generalized tendency to overlook wives because their paper trail tends to be smaller and more fragmented.)