D.W.
I personally LOVE sippie cups. I don't think that you have to ween kids from sippie cups because they ween themselves. They are great to use for road trips or when we are out doing errands. I use mine for water only because I believe most kids don't drink enough. Then you don't have to worry about if they spill, or get lost for a couple days. They get milk or juice at the table only. I don't think there is an age that kids have to stop using sippie cups, as long as they can drink out of other types of cups too.
So... just keep trying and she'll get it. Just like kids eventually get how to suck out of a straw. Just try again in a couple weeks or days.