Am I being unreasonable?

My husband has very little contact with his real mum and her side of the family. (We visit maybe once a year) They live 3 hours away. Yesterday he gets a text from his 16year old niece asking to come stay for a few days. I asked my husband to call and find out what's going on. Why she wants to come over after never bothering. I wasn't around for the phonecall but he said that it's just to see the children and that he was expecting her to call. About 4pm yesterday his niece texts saying she's booked a coach for tomorrow morning. My baby is a week old, I've got 4 other children. My husband went back to work when baby was just 3 days old. My mum and dad have been amazingly helpful taking my other little ones to school ect so I'm not without help. I just really don't want his neice over. I'm exclusively pumping milk so I'll have to either have to go onto formula or go hide away to express every few hours. I'm going to have to go meet her at the coach station. She has not said when she's going back. If it's after Monday then I'm left at home with Her... All that aside my eldest will have to give up her bedroom to make space. I don't even know her. I've tried telling my husband that I'm not over keen on her coming but he just says sorry and that there's nothing he can do now. Is it completely unreasonable for me to put my foot down and tell him to tell her she can't come? What would you do?