One of my favorite presents I've ever received is a silver frame from my sister engraved with "sisters by chance, friends by choice", and then she put a picture of us in it. I don't know how unique it is, but it was very special. Maybe you could do something like that, and then have a photo taken at the wedding of the three of you? That is all I can think of right now, but I thought I'd pass it along. Good luck!