Everyone is right...NO is the only option. And the bottom line is No, I'm sorry about your situation, but it wouldn't work. (Repeat as needed). If you offer an explanation or reason, she has an opportunity to talk you around it. AND really as others said, it is your home, she is asking you for something that is BIG and at your stage in life you don't owe this only-when-I-want-something relative anything besides your answer...which is no.
Sending strength and peace {{{}}}vibes your way!