Ok I have to put in my 2 cents. We aren't here to bask someone over the animals they love. We are here to help each other, right?
There are a lot of rat rescues who make you sign a contract before you take the rats home. One of the conditions is that you dont keep snakes in the same house with the rats. This is probably the reason she is saying there is a breach of contract and why she wants them back. Rat rescues are really kinda silly in my opinion, but to each his own.
I understand you love the rats and want to keep them, but it might be in your best interest to just give them back and find two more, not from a rescue. They charge way to much in the first place, and then you run into this sort of situation. I don't think its fair on your end to have taken them in and cared for them and paid for them, just for her to turn around and take them back. Then you're out money and heartache.
Anyway, I wish you luck with this and hope it all blows over quickly