How can I get my 15-month-old to drink from a cup? I have been giving her milk from a bottle, and juice and water in the cup. She doesn't take any more than a few sips from a cup. When I tried to give her only the cup she drank nothing for the whole day.
It sounds like your issue is less about nutrition than about control. Toddlers often resist giving up their bottles, having formed strong attachments to them, especially when they contain the ultimate toddler comfort food: milk. This can also mean refusing cups.
However, you are right in hoping for a bottle-free diet for your 15-month-old. Drinking from a cup develops adult drinking skills, sucking from a bottle doesn't. More importantly, drinking any substance other than water from a bottle promotes tooth decay of the front teeth.
So, aim to discontinue her bottles altogether by her 18th month. You can choose the more radical method of tossing all the bottles out. Though she may be mad and refuse liquids in the short term, no toddler has the will to resist the powerful thirst drive. When she gets thirsty, she will drink, from whatever is available.(Children also get a lot of water from semi-solid foods like applesauce.)
Another method is to slowly wean her bottles, from two, to one, then none, in a time frame you feel suits her temperament. Allowing only water in those bottles may make her less reluctant to give them up. Whatever method you are most comfortable with, it's most important to stick to the plan without any backtracking, until you are successful.