We had to help DD1 towards the purchase of a small car...as a veterinary student with lots of work placements in rural locations not served by public transport, it was more or less essential. DD2 is fine with this and accepts that her own course is unlikely to have such demands and therefore she's unlikely to get her own car any time soon, but knows that she'll have the use of our car when home and wanting to see her friends.Driving lessons too. It's swings and roundabouts. If the music course will help the younger child to fulfil her potential, and you can afford it, I can't see why the older one would have a problem as you would probably support her if an equivalent opportunity presented itself. University certainly isn't cheap these days!
It's occurred to me that this might be the kind of dilemma facing two-child families - if you do something for one you automatically feel you have to do the same for the other. With four children we know from the outset that it's impossible - we just do our best for each child at the time, depending on their individual interests and needs.