L.A.
I would "Kidnap" her and take her out with some friends.. You could pay for you and her and get the others to pay for themselves..
Make it " am a terrible sister for forgetting your birthday, But I will spend the rest of this year making it up to you" Party" ~ hee, hee
Just be super apologetic.. I bet she will forgive if you fawn all over her.. Make the time.. Get a sitter if you have to. No excuses on this one.