It’s not really possible to offer any meaningful, non-generic suggestions when all the information we have is the gender and age of the person you are buying a gift for.
Personally, I enjoy reading and I’d be really happy to receive a well-chosen book for my birthday. On the other hand, I know many people that wouldn’t be interested in being given a book. I’m 23 and female, and there are many other 23 year old women who wouldn’t want this as a present.
Is the present a joint one, from you and your boyfriend? The merit of the fact that you and the sister share a gender doesn’t really give you any advantage on picking a decent, personal gift – I think it’s something you’ll need to decide on with your boyfriend. I’d talk to your boyfriend and ask him about his sister’s interests, personality, tastes and hobbies, and then I think you’d be able to get something a bit more special. Obviously it will take a bit more time and effort to do this but I think that will show and make the gift much more meaningful and one she would be happy and touched to receive.
Best of luck finding something nice, I’m really bad at buying presents and I always leave it to the last minute, making it even worse!