T.F.
With a 14 and 13 year old, you'll have to do this again and again right around the corner, so you don't want to give the first born more than you'll be able to afford down the road for the others. Rather than a set cash amount (that she can blow on who knows what), make a deal to match her dollar for dollar on what she earns to spend toward a used car, and set a maximum limit. Take her to a used car lot (or look on line) to give her ideas of what she wants to get and so she can set a goal.