Most, men only focus on what's in front of them only, but not the future. It's very natural that your husband thinks your 99 will be nice to your BB. Have you asked him what would you guys do if your 99 just cannot take care of the baby one day (for whatever reason)? It would be quite messy then.
If your 99 doesn't live too far from you, she can come to spot check the maid to ensure the maid is taking good care of your BB.
Before my son was born, I thought about hiring a maid to help my 99 to take care of the baby (she said ok). But after learning the potential bad influence she has on my son, I changed my mind. I said I would want to see my BB when I came home from work, and we did not want to "bother" her that much.
I believe an open discussion with your husband is very important, especially make him think about the consequences when your 99 is not able to take care of the baby.