I'm concerned not only about your niece and her baby's safety, but also yours if he moves in with her. Like the other girls have said....
let it come from your landlord.
She's come from a difficult relationship and is still being controlled by him. Has she gotten any help for this? Has she seeked any sort of counselling? I know that you can't force it down her throat....i'm just concerned about her.
Take care. And try to talk to her again when things cool down.
I hope your landlord and you can sort this out, you don't want him anywhere near you, or her, or the baby 
