Answer: Environmental factors have more influence than genetic factor in this disorder.
Explanation:
Environmental factors have major role in this order, this is because the if there would have been genetic disorder then both of the sisters would have been affected with it.
Environmental factors is responsible for this disorder because although the environment in the home is same for both the kids but the outer environment might be different for the kids.
The external important also affects the kids and there might be some differences in the environmental conditions which affected one of the two sisters.