How do I (28M) feel about my girlfriend (26F) going on a girls trip that I don’t approve of?

My girlfriend and I have been together for just under 2 years. She had never been to new orleans before so we did that together last year and she loved it. Her and her sister made plans to go together sometime soon after that and her sister just booked flights for them for May. So now my girlfriend wants to go again with her sister. I’ve made it clear I don’t approve of this trip. I’d rather she not go. So how would I go about expressing my feelings to her about this? I’ve expressed my disapproval already by not being excited for her and now she’s wondering what my deal is because we aren’t really arguing about it. Just not excited about it. <br><br>TL;DR: my girlfriend wants to go to new orleans again with her sister, how to I express to her that I don’t like the idea?