Aren't you just teaching her that she needs to do what you say? Her motivation isn't coming from herself so it doesn't seem like a good lesson in persevering - at some point you will not be there or not be able to tell her what to do - what then? And for an anecdote I know 3 people who learned cello as a kid, and none of them play any music any more and pretty much stopped immediately after they left their parents home. Maybe a piano or guitar player could keep it up, but cello isn't something you tote to a house party to jam on with friends.