My mum tells people everything i say

You could try getting her more involved with the community, like to make new friends. It can't be healthy for her to be so isolated. Idk how your mother is about relationships with her adult children and stuff, but maybe you could try and be a friend to her?
Like call her everyday, ask her how she is, tell her about gossip at school, daily stuff that you wouldn't mind her telling your step-dad. This is to the purpose of pulling her away a bit from your step-dad and her little bubble.
Good luck with all of this! I see that you're hurting and I hope this will be resolved

Thank you for your reply, this makes sense and i will try this! : )