My sister wants to get high

And well... She says she is either going to smoke it with me or with someone else... I would rather her not do it at all... But she is going to do what she wants
But uh... I would much rather her smoke it with than have her smoke it with some random guy...

that will more than likely try to take advantage of her... Your opinion?

I'd keep on telling her how you feel about it and that you really would rather she didn't do it at all. You don't have to smoke with her just because she wants to smoke. I think you'd be better off telling her about all the dangers involved in drug use and the importance of moderation. If you're really worried about her then maybe you can accompany her without smoking yourself.