Tyler is almost 20 mo, and STILL isn't into feeding himself! I think he learned how to do it around 15 or 16 mo, and once the novelty wore off, it was "Mommy do." The sippy cup got tossed a LOT when he was your DD's age. We kept it available throughout the day for him to sip, and he started drinking from it more at mealtime around 13 mo or so.
I didn't do anything to "help" him learn, except give him a spoon when he reached for it. Same with the sippy cup...one day, he just started drinking from it. Since she sees you eat and drink, odds are that she knows what to do, but just hasn't gotten there developmentally yet. HTH!