What should I tell my parents?

Its picture day next week on Thursday. I wanna straighten my hair, but that's my sister's straightner. My parents are pissed because I cut my hair. Its against my religion to cut my hair. I felt like I was left out at school ( I'm in 8th grade). This is spring pictures. In fall we had picture day and it was for our ID so that's what I told her and I don't know what to tell her now. My mom checks my hair every day to check if I cut it. I have to do a regular braid to school every day. I don't want to do this on picture day so what should I do? My sister is really mean. She's like in her own little world. She's a senior.She is the one who straightens my hair because she thinks I might burn in because I never straightened my hair by myself. I'm in 8th grade. I'm 14 years old. My parents are so strict the don't let my go to my friends house. I really like this guy and I wanna impress him. My parents don't let me date but I date anyways but on my hair part i cant do this