How should I approach this situation that feels like a lose-lose?

I'm a situation right now where I feel like my choice is either to potentially be an asshole to a friend, or to not do something I really want to do. I'd love some advice on how to approach this.

There is an event coming up in my city. To get into the event, you have to buy a lottery ticket and if you win, you can bring a plus one. A friend of mine (let's call her A) bought into the lottery and if she wins, she told me she would bring me. I didn't think much about it at the time. I didn't ask when the event was or what it really entailed because it didn't even sound like a possibility.

But now another friend (B) reached out to me because she won the lottery. B's friend C also won the lottery. So B invited me and also offered me C's plus one because they were intending to bring each other. It seems easy that I should bring A since she had initially told me she would invite me. But the problem is it is my sister's birthday and she is coming to visit for the day, so I don't want to ditch her. Plus, my sister would actually love this event, so it would be a great way to celebrate.

If I invite my sister, it would really hurt A. I could not tell A, but she could still find out which would hurt her more. There is also still a potential for A to win the lottery at which point I would have to tell her I'm already going which I know would hurt. All of this would make me such an asshole to A. Option 2 is to not go and enjoy the plans I already had with my sister. It feels like the right thing to do but I kind of feel like I'm letting myself down by turning down an event that I'd like to go to and a great way to celebrate my sister's birthday. I also have a tendency to people please so I'm worried I'm making that decision based on others and not myself. What are your thoughts? What is the best decision here? How should I approach this? Should I say anything to A?