HELP! Complicated situation

Tomorrow I start my first day at a new job. I really need this job and I wasn’t planning to have any conflicts with my schedule. But now im in a complicated situation

My sister has decided to get married on Friday and Im scheduled to work. She was originally going to get married next summer but has decided to have a quick wedding with immediate family. Im supposed to be the maid of honor. Obviously its my sister and theres no way in hell I would want to miss her WEDDING. But this is my first week at the job and there’s no way I can take a day off so soon after starting. And i really need this job. Im a single mom and dont get support from the father. It was so hard to find a job and I dont want to be fired. But on the other hand its my sisters wedding. What would you do?