You properly don't include any mention of gifts at all on an invitation. The etiquette error is in the assumption that you're expecting gifts in the first place. And while it's generally a safe bet that people will bring gifts there is no polite way to direct people how to spend their money, or in this case, not spend it. (This is also why you don't include registry info.) Your sister is (obviously) not concerned with gifts and would rather just have the people she loves there with her. It's very refreshing, so many people get hung up on the presents.

I'm with her, I'd rather have the people I love with me and I don't really want gifts at all.
The only proper way to get this info out to people is by word of mouth. Or, if she has a wedding website, the info may be posted there. If it's really important and she doesn't care about "proper" then I agree with another poster that this information should be included a seperate instert than then invite. There are going to be people who bring gifts regardless. They want to express their happiness for your sister, and a gift is a natural way to do so. Best of luck!!