Hey there Emily,
I'm sorry that you've had to go through this, but I'm glad that you at least have your supportive sister on your side. It's unfortunate that your mother reacted in that way, but it may not have to prevent you from telling the people you want to tell. As Holly said, you could tell your teacher that your mum already knows, but is unwilling for you to tell others about the sexual assault. I'm not sure how things work at your school, but at some in the UK, they will have a guidance counsellor who is bound by confidentiality to you. For others, the nurses at school (provided that they are under NHS), have that same duty of confidentiality. Therefore, you can open up to them and ask for possible referrals for counselling, if possible.
I hope this helped somewhat and feel free to
PM me if you ever need to talk!
Kyra