My heart goes out to you des in this situation....
You have no reason to feel guilty or dirty or wrong!!

You & your step dad know the truth & I am so sorry that your mum is so caught up in her own world to realise that you are being honest & truthful with her!!!!

You can talk to me anytime you need someone to listen!!!
YOU HAVE DONE NOTHING WRONG, so do not punish yourself over your mum's behaviour!!!

{{{{{{{{{{{{{{{{{{{HUGS}}}}}}}}}}}}}}}}}}}}}}
Flinty