Step-by-step explanation:
In this case, I would have stick to my commitment which I made first.
No doubt the invitation of the second party was just like a dream come true, as that was being held by a popular group and all of my friends were going there, but ethically and morally I am supposed to stick to my commitment which i have made before this invitation. So my decision will be on moral and ethical grounds. I would attend the sleepover birthday party and would hope that the other party of popular group happens again.