Thank you for talking about this with us. Your mom has put you in a very difficult position of having to keep a secret — which isn’t good. The anxiety and tension that this has created certainly isn’t fair, and you need some support.
You didn’t mention your sister’s age and I think it would be difficult for her if she were younger. It also sounds like it may be difficult for your dad — and if your mother has been so careless he may already know. I think the first place is to get some support outside of your family. Your school counselor is a good place to start. He or she can help guide you with next steps. You have done well speaking about it here. I would take the next step and talk to a professional who can guide you.