OMG she is NOT a big kid, she is 12 - that is nowhere near being a big kid. Have you any idea the emotional turmoil a 12 year old is going through inside? She may not show it but I guarantee she is the most confused right now than she ever will be in her life.
People say the teen years are traumatic but in truth its the transition from being a "child" into a "teenager" that is the most traumatic. Right now, she is relying on routine and memories to get her through the day and that includes being able to have capri sun drinks.
Her routine will be providing her a comfort and a safety net to help her get through what is a extremely confusing and traumatic time and you are taking that away from her by demanding that she be a "big" kid, something she has no idea how to do.
Have a heart and show some sympathy, drinking capri sun isn't going to hurt her at all as long as she's not drinking tons of them but having something she likes, something she's always had, something that stays the same when the whole world is changing around her might just make the difference between having a happy kid and a chronically depressed one.