Learn Spanish (Latin America): Sus llaves están en la casa.

In Spanish, possessive adjectives like su and sus have to match the thing being owned, not the owner. Because llaves (keys) is plural, you must use the plural form sus, even if they belong to just one person.

Does sus always mean "her"?

Not at all! Su and sus are very flexible. Depending on the conversation, they can mean his, her, its, their, or your (formal). In this sentence, we know it means "her" from the English translation, but if you were talking about a man, sus llaves would mean "his keys."

Why does the sentence use están instead of son for "are"?

Both estar (están) and ser (son) mean "to be," but they are used differently. We use estar when talking about physical location. Since this sentence is about where the keys are located right now (en la casa), están is the correct choice.