Oh dear, this is out of order & must be suffocating.
She may own the house but it is your home.
Next time you’re over at hers, try sorting out her cutlery drawer, saucepan cupboard! See how she likes it. I’d hope she takes the hint.
Can you ask your DP parents to visit you so that when she comes over she’ll feel like a bump on a log & intrusive?
Would you feel able to change the locks? It’s a bit drastic & in reality, as your landlord she should have keys, (for emergencies) but she is usurping all the usual landlord rules so you, as a tenant, could usurp the tenant rules and change locks without recourse to her.
You could be devious, get locks changed & say there’s only keys for you & DP & be very very slow at getting extras cut?
You could at least put a chain on the door! You say “Not today Mummy” as she pushes open the door & gets stopped in her tracks!
Tenancy rules are that you are “entitled to the quiet enjoyment of your home” so everything you describe is tantamount to interference, control and utter rudeness.
Can your DP have a word with her & Nan? Head of the household & all that!
Good luck with your confinement & new baby.