Vet visit. Then examine the circumstances under which she growled (you do not want to end the growling - you just do not want biting - growling is communication). For instance what do you mean when you say you "grabbed her collar"? I would avoid doing that and use a command instead - for instance - teach her "In" as a "go into this area" command. I try to always reward compliance with "in" as well as using treats when I am training it. (One dog has "in" as a kennel command; the youngest's word is "load.") I would also make it so that people like your MIL did not tell the dog what to do.
Good luck with your girl. A mixture of new to you dog, young children, MIL can be hard to manage.