Kind of sounds like the Spock approach. Logical. But if you are a Star Trek follower like me. You know the logical Vulcan approach doesn't always work out so well.
If it were me I might give it one more try. Logically showing up at 10 and leaving by 2. I'm sure you could find other things to do in the afternoon close to the beach. Then maybe do it again the next day and go home. That way everyone gets the best of both worlds.
Both of my parents are long gone. And once they are gone you think back and think (I wish I would have). If you give it another try and she still "uninvites"you. You won't feel so bad,because the blame is entirely on her wanting to "party" instead of being with you.
Just my opinion.....
Fred