I need info on Mocking Birds?

My sister found a fledgling Mocking Bird in her yard a week ago after a bad storm. It was the only survivor of three. There was no nest around to put it back into so she took it in and began feeding it. It's doing rather well but there is a problem. Since being brought in and being around humans, cats and dogs it no longer has a fear of any of these things. No one is sure whether it would be best to keep the bird in a home (I would take it in) or to try to release it. If she lets it go the first time it plops itself down in front of a stray cat it's going to be done for. Which is better keep it as a pet or try to let it go and hope it can fend for itself. No one wants to be cruel to the bird which is why we aren't sure which is better.