Yes, birthday stickers can be obtained from City Hall. Or if you are staying on-site they give you a birthday button (and balloons and an autographed Mickey & Minnie photo).
But by wearing the sticker, all the CM's (and a lot of guests) will wish your sister a Happy Birthday. For my Mom's birthday in December she got a birthday dessert (I can't recall what it was) at the Blue Bayou and at Ariel's Grotto in June, they put a candle in my dessert and sang for my birthday. I'm pretty sure pretty much any sit-down restaurant will do something. If you make a priority seating reservation prior, make sure to tell them it is your sisters birthday!
Otherwise, at the Plaza Inn, they do Happiest Birthdays on Earth. I'm not sure what the minimum party size is, but heres the link:
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