It is close to halloween so you have an out... Show them a photo and ask them if they know who it is? Tell them you dressed up and she what their reaction is. If they say you look like the sister or daughter they never had then tell them more...if they are in shock maybe back off. I've not done it this way, but I did show up at my parents home fully dressed one halloween and they were in shock at first, but then started joking with me and telling me how much I looked like a cousin of mine. Good luck.
