How are you going to feel if you receive something that could improve yyour financial life as a birthday? Holding this asset(Bitcoin) for few year from now can become a game changer for her when other gifts might have lost value.
It's always nice to receive gifts from others, so i would be very happy and enthusiastic about receiving bitcoin as a gift (maybe because I'm already a bitcoin adopter), although I also like when people give me a physical product, instead of money. So I know they have taken their time to search something I would like to have and receive in a special day. In the case of someone who isn't familiar with bitcoin, I think you need to create a bitcoin wallet yourself and give your friend and sister access to the respective wallets later, once they have understood how it works and how they can move satoshis between addresses.
But keep in mind they might develop interest for it or not. So in negative case, you will have to elaborate a solution on what to do with these satoshis.