Rant required!

So my husband has told his 10yr old Niece that she can come and stay at our house in the summer holidays (they live away so we don't see them often, I get that) but he's never offered it before - but does so when iv literally just given birth! We have a toddler too so I'm gonna be manic come the summer! He also had this conversation with her when I wasn't there and I found out by him telling me she may be coming to stay, whilst she's sat looking at me. How could I say no? I mean i didn't say yes either tbh. He just keeps saying "let's see how the baby is first" - never mind me. Recovering mentally and physically, raising a toddler, with a new born, breastfeeding and going to be genuinely a bit all over the place. His brother doesn't help either, texting us days later like "she's said she's sp proud that her uncle has asked her to stay, she thinks she can help" So in other words - don't now tell her no cause you'll break her heart. Just to clarify too- she can't "help" she's 10. I let her "help" with bath time while she was here the other week and obviously it was just playing with the toys and rinsing bubbles off. If anything it prolonged bathtime- which is fine, but I can't have that for a solid week whwn iv got 2 and recovering from birth. How can I politely tell my husband that it is not an option, she's welcome to visit but not stay with us. I don't want any visitors in the early days staying more than a few hours max tbh cause i want to be able to rest!