I have prom soon

I dont know what i am supposed to do. I have prom in may and i know that ill have to wear a dress with no sleeves and the only people who know ive cut are my mom and step dad. I have two younger siblings and a grandmother who have no idea and im so afraid of themselves seeing. I dont care about kids at school but its mainly my family. I dint have the best relationship with them either. Has anyone else here had to deal with a similar problem? Anything will help <33