I'm going to tell my girlfriend not to bring anything while meeting my family
Why don't you want her to bring something?
She always brings too much things, so I want to give her a lesson on how to travel light.
Well, it is her decision whether to follow your advice or not. I think she will just do as you say since she likely is nervous.
She better listen to my advice or else.
I don't think you should be telling her what to do when it comes to this. If she wants to bring stuff then let her do it, it's not a big deal.