Second Posies suggestion.
Only perhaps add at the bottom of the list 'And don't come back.'
Harsh? Yes. Here is the most worrying of all the sentences you have posted:
'I do feel like I have a duty towards her.'
You don't. It sounds as if this woman pretty much destroyed your childhood, and the repercussions are something you clearly deal with every day. Now, in her old age, she leeches off you to provide absolution - if you keep in touch with her and let her play 'mummy', she can't have been that bad, right? Only, horrible, horrible person that she is, she still can't actually treat you with proper love or respect.
So, you don't have any duty to her. None at all.
What you will have, however, is an absolutely MASSIVE duty to your baby. A duty to make sure that the kind of atmosphere never poisons their development. And a duty to yourself, to heal more than you have done, so that your young family can thrive properly.
It doesn't sound as if you have the equipment right now to do anything other than endure her visit. That's fine, you'll get through it. But please, once this horror story has gone home, get yourself into counselling asap. Find the tools to do the right and best thing for yourself and your family and get this happiness-sucker OUT of your lives.
Don't let her horrible influence affect your family and especially the precious time with your new baby. Don't let your family have her hanging over you any longer. Get rid.