How can I make her let the rabbit go?

It is your home tell her bluntly. But kindly. Tell her the mother of the baby sits in the rain waiting to find the baby. She probably thought she was doing a good deed....and she loves animals, so maybe this would work. BUT there is a chance after you do this the mother will not return. And you will be the new mommy. There are many things to know about baby bunnies. The older rabbit would most likely kill it.