[Hakulyte]

1. Finish high school. It will increase your chances to find work.
2.a) Inform yourself of what kind of work is available around where you live and take decisions based on that. (Plan A)
2.b) Check out what majors you can take in case you would continue studying in a field that you actually care more about. (Plan B)
3. Get a driving license when you can, but only get the car when you will really need it. @ work (unless it's super close?)

Why do you think you're not paying attention as much as you may have before?
Why are you not motivated as much anymore?
Is there something you can do to help yourself keep interest?
Of course, most of the things you're learning will not be very useful in the end, but the experience, the wisdom gained, the opportunity to be in an environment with the rest of society and doing assignments to challenge yourself are all very helpful things to help you develop yourself as a person. If you're still not sure of what you want, you may as well keep absorbing this "random general knowledge" for the sake of graduating and having an easier time in interviews for work etc. It benefits no matter what you do.
Also, you shouldn't compare yourself to your sister or even anyone. You should focus on finding what matters for yourself.