Personally, I never like to encourage keeping fosters specifically because it makes it hard to help others who may need help just as much as she did. However, every case is different. Reminds me of my last dog, she came from a puppy mill and was absolutely terrified of everything. She wasn't a foster, I had been contacted about her through some rescue pals. After hearing her story, I did everything I could to make her feel like she's safe. To make her know that this was going to be her forever home. It took her about a year to fully come around. She became comfortable with the groomer and new people. She even stayed at my sisters house when I went on a cruise and did fine. She was comfortable around new people.
I loved that dog so much, and she learned that not all people are bad. I would think little Molly would be able to learn the same given the right amount of patience and time. Of course choosing a new home for her will take longer and in the end it's really up to you.
As for my own dog, well she passed away about a month ago she had heart problems due to being so poorly bred. She helped me learn that every dog just needs a chance to come around, and I helped her learn that she can be loved and it's okay to roll around in the grass.
Anyway, the point of what im trying to say is that if Molly is given the right circumstances, Im sure she can come around to people who are just as good as your family, but ultimately it is your decision to make. Just my two cents
good luck and so good to hear about her coming around. It's a beautiful process to witness