I fall kind of in the middle on this: I don't think you need to pre-plan it entirely with your sister, but asking her what her favourite colours are and then choosing one might not be a bad idea! That way you don't need to tell her what you're making, and you can pick a pattern that you like, but you can be guaranteed that she'll at least like the colour.

I see what twistedchick is saying, that it's something you want to give them of yourself, but at the same time you want it to be something they'd enjoy and use! And if you know it's gonna be a big Thing asking what colour she'd prefer, it's also totally valid to not make anything either. Lord knows I've ended up doing that in some occasions where it was guaranteed to be a pain in the ass to involve the person, heh.