You have done a good thing by taking in a stray however
legally you can be in for a world of trouble. I would recommend you go about this the right way instead of hiding her from her family(I am using this term very loosely). Of course you don't want to see her go back to the situation she was in but what you are doing is breaking the law. I just came across a situation like this myself and had to surrender the animal to the rightful owners. As hard as this is, it is important to uphold the law. I seriously recommend that you contact your local humane society and follow the proper procedures to adopt this girl. Trust me, in your heart I know how you feel but in the end it will be worth the peace of mind to do it properly. Have you had the dog vetted yet? If not, make sure you get documentation from your vet as to the condition and any health problems the dog has. Check to see if she is microchipped, etc. Legally animals can be taken away for cruetly and then adopted back out...I am sorry you are having to battle with this and in the end I hope you win but I think it's best to do the right thing and seek out legal help.
On another note, if she did not have a collar on you can take her to the nearest animal control and they are not required to hold her for any length of time for her owners to claim her. She can be immediately put up for adoption. You may want to take her in and let them know you found her and then turn around and adopt her ..although you may have to pay for her, it was done legally

and there can never be any questions asked!