Is this normal for an 18 month old (my great-nephew)?

I think you should let her worry about it. If she has a problem with it, she will change it herself. Leave her alone. She is not hurting the child, and he will wean eventually. I know this won't be a popular answer, but a child's brain develops quickly until the age of two, and milk is the better for him and more tailored to his needs than cow's milk.