I don’t know how to say this kindly but I do mean it kindly, OP, for you and your daughter, because you have the best intentions for her.
But you are not listening to her. You are focusing on what you want her to enjoy and not what she would actually enjoy.
She wants to play the harp, which is nothing at all like the viola.
She can see her sister loves playing in orchestras, but she is not agitating to do the same.
Learning any instrument involves boring mechanical repetitive work in the first years. Those scales! Those bowing exercises! The frustration when it does not sound the way you want it to! And viola parts in orchestras are not easy. You have to do the practice. You need something that makes you want to keep going. If you do not really want to play the instrument, or to play something, anything, for the sake of playing with others, then what is it?
The viola in an orchestra is also a state of mind. It’s not all about great tunes, though violas get them too. You have to love being in the middle of the harmony, and changing the colour and character of the music. Is this what your daughter loves?
You cannot force the positive experience you want for her if it does not chime with her.
There are a whole lot of practical reasons why most of us might have to say no to the harp. That doesn’t mean imposing a different instrument on her is the answer. She may never come to see the same advantages in it that you do. And then what’s the point? What would she like to have done with all those hours?