It is hard sometimes when a situation lands in our lap to know what to do for the best. Bringing a cake to the party for someone who had taken time away from their special day to celebrate with others was a lovely gesture and reminded your aunt that she was still important and not forgotten. This gesture would have meant a lot to the recipient.
JWs are hard to please as they accept no compromises but this is a case when they needed to take a step back and allow others to have their way. I would have made an announcement stating that a cake was being prepared and candles lit giving any JWs present a chance to make themselves scarce whilst it happened. what they chose to do was then up to them. Sometimes they need to learn it is not always about them.