I don't know,that's a pretty old set and I don't recall seeing that ring in any others. I've seen her wearing various rings on various fingers though. Lot's of non-married and/or non-engaged people wear rings on their ring fingers. Not saying you're wrong, just saying there's not enough proof to say she is as if it's a fact.
Edit: Just doing a quick look through some old sets she wears that same ring on her left index finger in the "dirty maid" set.