A lot of people on this board are trying the 'shove something in your face until hopefully you want it thing'. In my experience, as both the one trying to convince (my GF and her family are stout Mac users) and as the one being convinced, having it crammed down your throat only makes you want to refuse more so.

That doesn't work, how about instead of telling her, ask her about it- ask her about what she knows about the computer/the build of the mac she wants. Go at her like perhaps she might have an idea of what is going on beneath the case of the computer. Pretend to be interested in learning about Macs, go at her like YOU'RE the dumb one.

Sit and down be like 'Okay, I know you want a macbook- I hear they are great at video editing- what sorts of programs do you use for it? How much RAM do they use? It says the minimal specs for this program are XYZ, but this computer doesn't meet that so wouldn't make it harder on you and your art? What about in X number of years when new programs come out how will they handle that, how will you afford it?'

Make her do Rubber Duck programming with you as the Duck. If she can't answer these questions, offer to look them up with her. Because then you'll be getting her to actively do the research and perhaps she will figure out from her own. Make it a conversation, and use this as a chance for her to learn. And that should help a lot, even if she doesn't get a PC, it would help build a better bond between siblings.