I'm thinking of offering to pay for my sisters holiday, and then taking back the offer
Don't you want to do it for real? How do you think she'll react to this stunt?
She'll be upset for a few minutes, but then she'll be fine.
I don't understand the purpose of extending an offer that you intend to withdraw. What if she makes plans, clears her calendar, and spends money preparing for the bogus holiday vacation? Do you really think she'll get over in "a few minutes"!?
She won't do any of that since she knows I'm her brother.
Even if she knows you well and suspects you are joking, it is still kind of cruel to pull such an act. Why not surprise her this time and actually go through the offer? She'll think you are so sweet.