No, this is not right, if they want to see a warranty deed, they can go to the court house, heck your niece and go to the court house and get a copy of it, it is public record.
It shoulds like the mobile home company is trying to get a lien on the property also.
Many mobile homes go onto rental property and the such. And if they need to repo they can come there, at most they would need you to sign a statement that they can come on the property.
I would say you need to go and talk to the mobile home company yourself.
Also you need some sort of legal agreement with your niece, if they start selling drugs, or start having junk cars setting around and the such, you have to have a way to evict them off the property if you need to.