To file or not to file custody case is upto you taking into consideration of other circumstances, which may develop after filing the case, i mean, the relationship with husband, in-laws. As far as child custody is concerned, your case is clear. you will win the case absolutely. The law says that till 5 years mother is the natural custodian and later on father is the natural custodian. but in the case of girl child, most of the times, the courts tilt to give custody of the child to the mother after those 5 years. only after 5 years age of the child, in some cases, which are very few, the father may get custody and in most of those cases also, the mother gets the custody.
But if the dispute is between the parental grand mother and natural mother for the custody of the child, difinetely the courts will see the welfare of the child in the custody of the child. So, you can go ahead in claiming the custody of the child, but keep in view of the adverse atmosphere may develop in the relationship between you and your husband.
wish you best of luck.