I don't think it is necessary to do anything other than wish her a Happy Birthday when you speak to her. If you and your new husband (congratulations, by the way!) want to, then getting a card for her and arranging for this to be left at her place setting, or given to the the evening before, would be a nice gesture, but is not necessary.
Does your fiance normally send his aunt a card or gift? If so, then he should do so this year as well.
If your fiance wants, he could say happy birthday to his aunt in his speech but personally I would stick to doing so when you see her (in the receiveing line, if you have one, r when your are circulating and speaking to guests)
It's not out of the realms of possibility that there will be one or two other guests who also have birthdays or anniversaries and singling her out might embarrass her, and make another guest who isn't mentioned feel overlooked.
I would only mention it in the speech if Aunt is doing something special for you (so if you'd be thanking her specifically anyway, and want to add that not only did she do X,but she gave up her birthday to do it) or if she has a very special, close relationship with your fiance.
If FFIL is likely to make a huge issue out of it I might go as far as to get a small gift for her and pout this on her place at table, as well as the card, just so if he brings it up you can respond with a "Oh didn't you notice, we did include it, we spoke to her personally and she had a gift and card"
If I were in aunt's place, I would not want it raised publicly - I would feel it put any other relations who had not mentioned it or given me a card/gift feel uncomfortable and if I were a guest it would strike me as a bit odd.
