[Ambedo.]

Social anxiety can definitely make going to new places an incredibly difficult thing to do, especially when you don't have someone you know well to go with.

As others have said, you don't actually have to do anything that you don't want to do. Perhaps you can talk to your sister about the trigger that attending with strangers will be for you and explain that, while you appreciate the gesture, it's in your best interest not to go. If it's something you're able to do, you can also offer to pay her back for the ticket or help her find someone to sell it to.

If you'd still like to try to go to avoid addressing things with your sister, perhaps you can start small. Depending on how long you have until the event, you can try taking baby steps to help ease you into feeling more comfortable in social situations. For example, you can start out going to a familiar, fairly uncrowded place by yourself a few times until you feel comfortable doing that. Then, you can try going to a familiar, more crowded place such as a mall and repeat the process. This is a process called systematic desensitization and can actually be really helpful when dealing with anxiety!

At the end of the day, the most important thing is that you're making the choice that YOU want to make! You have to take care of yourself first, after all.

Take care!