I'm sure the translator knew that Nance was a shortened form for Nancy; otherwise he couldn't have translated it as parienta, a rather colloquial form for wife. The word pariente meaning relative is almost always used with the -e ending, it has no feminine form except to refer to la parienta:
- m. y f. coloq. Esposo con relación al otro miembro del matrimonio.
In this sense it is almost always used in feminine. I have never seen "mi pariente" used to refer to someone's husband, at least in Spain.
So, why using parienta? Because literary works are not translated, but adapted. If I had to choose between "vete a casa con Nancy", "vete a casa con tu Nancy" or "vete a casa con tu mujer", all the choices are correct but there are slight difference in meaning:
- "Vete a casa con Nancy" sounds conforting, and it implies that both people know Nancy closely.
- "Vete a casa con tu Nancy" sounds a bit despective towards Nancy.
- "Vete a casa con tu mujer" sounds between neutral and conforting, but it implies that the speaker does not know Nancy in a close way (maybe he does not even know her name), which might be the case in the novel. Could Nance be used in the novel as a generic way to refer to someone's wife when you do not know her name?
And using parienta is just because it is a very colloquial way to say wife, and it fits very well with the low-level language used in the village where the Tom Sayer novel takes place.