Well, it's not going to be easy so you are right to ask these questions before she arrives.
It's not a fun job to work with a mother or father working from home, but in your case you'll be on maternity leave so it's even worse ...
Everybody will need boudaries and you can make this experience a success.
If I were you I'd make her responsible for the 2 older children from the beginning. And when they're in school, ask her to be in charge of the baby so you can get a break. Go out, run some errands, go to the gym, see a friend.
Get any chance to get out of the house. You'll feel better and she will enjoy being on her own as well.
It won't work out if you're always there monitoring what she does.
Good luck
