How would this sentence differ from "They show me your house"?
It doen't differ. But your translation is better than Duolingo's. Actually, it is more natural to say "Eles me mostram a casa deles" if you want to say "They show me their house".
It should probably also accept, "They show me his house" since that's a perfectly natural thing to say. It has multiple translations, the correct one could only be inferred by context which is lacking in this sentence.