A.A.
My neice went through the same problem. Would drink other drinks from a sippy cup, but not milk. For her, she was breastfed and would only associate that with milk, not a cup. Her pediatrician finally said to only give milk in the sippy cup and nothing else. Eventually she would cave in and drink it that way. It only took 3 days. She was not dehydrated or anything and has had no problems since. The pediatricain called what she was doing a silent protest and said she would give in when she saw that the milk was going to be in the cup only and that until she drank the milk she would not get any of her other drink choices. Maybe this will work for you. Good Luck!