Our niece is doing the same, she said that there is a girl who sits on the stairs & that is who wakes up baby. Our niece lives with us & is 5 so we just thought she might just be talking about her imaginary friend. Our son is fine when we stay away, he is in his routine. Could this be a faze he is going through?
Offer him verbal, physical and emotional reassurance but try to gauge when he needs all three. Trial just a simple “you’re OK” when he starts to rev up and reserve the full on cuddles and comforting until he really needs it. Can you engage your niece to do the same thing with him? His behaviour probably is due to the change of environment and scenery and perhaps he’s picking up on your business and change of focus as well.
Give him time to settle and reflect the rest of the family’s adjustment to your new home.
Be patient and kind with him, this will pass.