I'm upset over a future stepsibling who will be 17 years older than me.
Why does the idea of an older stepsibling bother you?
Because I want to keep being the oldest sibling in the family.
Your future stepsibling has done nothing to you except be older. Being jealous of them is unwarranted. Let go of your status in the family and accept your place as being younger than others.
I don't want to.
It is not good to be so hard headed like this. Things change all of the time, and your step sibling cannot control their age. They have done nothing wrong to you, you should not be wary of them before meeting them.