Some parents never grow up.
As in, they never develop and evolve as their children do and remain in adult / child mode forever.
Especially if they themselves had a shit childhood which sounds like the case. It may be she went through things you don't yet know about - I learned something 😬about my mum's childhood only recently.
So they never learned appropriate behaviour, boundaries and response. It's incredibly sad.
You have the opportunity to break this three generation pattern.
I know you're afraid of her reaction. You need to adult up, too. Maybe some therapy and assertiveness skills training could help you.
Your OH is quite right to want your privacy as a couple.
Practice your clear, calm statement to her.
'Mum, we really do appreciate your help and your time with DC. Please don't forget that our bedroom is our private space - if the door's closed, it's private. Now, do you fancy a piece of this cake?'
Then if it happens again, put a lock on your bedroom door.
(Why there wasn't a lock on your childhood / young adulthood bathroom may be a conversation for later.)