"She dances in her room."
Translation:Ŝi dancas en sia ĉambro.
Interesting, so we would still use sia if we were talking about a guy? "Li dancas en sia ĉambro."
Well, that could be the English translation as well. It is only context that makes us assume it is her own room. Anyway, I learnt something.
Accusative is not used, because there is nothing, which could demand its use. After prepositions like "en" or "apud" follows usually the nominative.