Wouldn't it be easier to just borrow the car and pay for her sisters plates and put the car on your insurance? I would think if she wrote a letter stating that you are borrowing her car any police officer would be fine with that if you get pulled over.
How can we put it on our insurance if we?re not the registered party?
“Make men large and strong and tyranny will bankrupt itself in making shackles for them.” – Rev. Henry Ward Beecher (1813-1887) US Abolitionist Preacher