Girlfriend wants me to take her sister on canceled time

I scheduled a really nice date which involves prepurchased tickets and is kind of intimate, my girlfriend suddenly feels ill and suggested I take her sister. Getting only dated for a few several weeks, I said no nicely because I thought it would be weird barely knowing her cousin, and I would rather go with among my good friends. She is insistent on my going with the girl sister to get to know her, but I feel there are much better ways to do that. I feel such as it’s perfectly acceptable to consider my good friend instead of her sister whom I hardly know on a pricey and intimate outing. I don’t understand what the acceptable approach is certainly.

TLDR: Girlfriend wants myself to take sister, I want to take homie on canceled time.