Q. Why is it that my toddler drinks water and juice from a sippy cup with no problem, but milk only from a bottle?
A. I hear this question a lot, and I think it's a comfort and habit thing -- many children associate milk with the bottle and don't feel right drinking it any other way.
Just keep offering her milk in her sippy cup, even if she doesn't actually drink a lot of it. Children don't need all that much milk by the time they're toddlers -- 16 ounces a day is fine. And if she has other sources of calcium and vitamin D in her diet, such as yogurt, cheese, or fortified cereals or orange juice, it's perfectly fine if here and there she drinks less milk.
If you think it's a comfort issue, try snuggling with her while she drinks milk out of her sippy cup, and she'll be more inclined to use it.