Deed of agreement/guarantor loan

I'm in the process of applying for a loan with my mother in law as guarantor. She currently has a mortgage with NAB and has plenty of equity. My bank is telling me that we won't get unconditional approval until NAB agrees to a deed of priority over the MILs property which can take as much time as NAB wants and they can decline it. I'm just wondering if anyone has been in a similar situation and how long it would normally take for a deed of priority to be set up. Also what would be involved from the MILs point of view in terms of her mortgage with NAB? Does she need to provide any paperwork or documents or is it just an agreement between the two banks?

I have just done a loan with my fiance parents going guarantor. They could only do it if they owned their house. Have you checked to make sure thats not the same for you?

Thanks for the reply. They said she just has to own 20% which she does. But it doesn't matter now anyway. Coz she has decided not to do it.